Here are some tips for teaching children Arabic as a second language (or learning it yourself):

1. Be dedicated and ready to fail

Dedication is the key to succeeding at anything. You will have to dedicate time and money to this effort.

One of the biggest mistakes I made with my children was being afraid of failure. This lead me to not teach my children anything for a while because I feared that it would be incorrect. To get past this you have to set goals for yourself and your child, and realize that if Arabic is something important to you then your child with mistaken Arabic is better than him with none at all.

2. Make sure your child has proficiency in the skills of their first language before introducing them to the same in Arabic

Many times when we attempt to teach our children another language (and we ourselves are not a native speaker of that language) we often expect them to pick up skills that are beyond their ability at that time. For example, if a child is not proficient in writing in the English language, then he or she will probably not catch on too quick with writing in Arabic either. Instead give your children some coloring books. Coloring helps the child build the motor skills they need to handle a pen or pencil, and in the end they will be better writers.

Don’t expect your child to learn the Arabic alphabet if he has not mastered the English one. What I mean by this is do not introduce more than one of the same skill set at the same time, not that you should what until he has mastery of the English language in its entirety before teaching him Arabic. Trying to master the small skill set in two languages at the same time will confuse the child and you will probably be wondering why it is taking so long for them to learn.

3. Make use of technology and Audio/visual materials

If your child is a toddler, then I highly recommend that you get some Arabic cartoons or children’s movies. I do not recommend anything from Disney, most of what they have is in the Egyptian dialect and even if it wasn’t contains way too many religious and sexual innuendos. Some classic cartoons like Loony tunes have been translated into Arabic. Some countries have begun their own production, and some other classics have been translated like “The adventures of Tin-tin” and “The Bernstein Bears” both with impeccable Arabic. I saw my children’s Arabic skyrocket after they starting watching these cartoons, and credit goes to my wife on introducing this to the kids.
Look for the JumpStart Toddlers programs in Arabic as well. “My first 1000 words” by Obekan is good, as is “Qamusi al-Ajeeb”. We have used only the last two; my children highly enjoyed them and learned a lot of vocabulary from them in context.

For older kids (maybe from 7 and up) :

The University of Medina Arabic Program has now been professionally printed in part along with an English key and the whole course is interactive on-line. (use Explorer not FireFox for this link)
Other people have used “Al-Arabiyyah bayna yadayk” with its books and CDs.

Rosetta stone and WorldTalk have programs as well.
4. Design activities for learning

Try to make learning Fun. Use activities like the ones found here at Educating the Muslim child. Another thing you can do make tags and tag everything around the house, and then constantly ask each other the names of these things. Randomly ask your kids something, and tell them that if they answer you in Arabic they’ll get a dollar. Incentivize their learning. I have a friend from Brazil whose father was so keen on him learning Arabic that he would take him to Arab families and drop him off for the day. At home his father would pay him a little every time he answered him in Arabic, but if he answered in Portuguese he’d get nothing. He grew up speaking Arabic and thinking that the only reason his family did not pray was because they were irreligious. On his 17th birthday when he went to Lebanon to visit distant relatives, and to his surprise, he found out that his family were actually Christian Arabs, not Muslims. He chose to remain Muslim and actually went on to pursue a higher degree in the Arabic language and Islamic studies.

5. Immersion

Try to have your kids visit families that speak Arabic and mix with their children. I have even heard of people going to egypt for the summer and having their kids live with families. If you can’t do one of these then…
If your children are older, go on an English fast. Set a time of the day (preferably longer than an hour) in which your children (and yourself) can only speak Arabic to each other. Regardless of your level of vocabulary. Don’t say “I don’t even have enough vocab to do that myself, how will I communicate?” Have you ever seen children have any trouble getting what they want because of a lack of vocabulary? One advantage that you have as an adult is that you can always pick up a dictionary and learn a few more words as you like to use with your kids.

Buy Arabic books, AND NOT JUST RELIGIOUS ONES, and keep them in easy access around the house. I say not just religious ones because the Arabic language is more than Din, and you need a broad vocabulary to be able to express yourself. I’ve found most students here in Medina that only concentrate on religious texts to have only a superficial command of the Arabic language, and their understanding is stifled and usually leads them to misinterpreting the faith as a whole (a phenomenon I really think deserves to be studied).

Buy cassettes of Arabic poetry as the adage says “Al-Shi’ru diwan al ‘Arab” Poetry is the record of the Arabs. Poetry helps the mind develop and gives added proficiency in the language, as most classical works of literature in Arabic are poems.

A small set of Arabic readers is good also, we use the ones written by Muhammad Muwaffaq Salimah.

For exercises (this is for more advance students) we use “Silah al-Talmidh – al-Lughat al-Arabiyyah” from Egypt. Ask most Egyptians and they will know exactly what this is. It has a lot of vocab and a lot of exercises on each lesson, a very helpful series, it goes from K-12.

6. The Quran

Take your kids to a school that will teach the reading and writing of the Quran. Preferably not a “catchabeatdown”  Madrasah;  it traumatizes and puts a mental block on learning. The Quran has been key as an educational tool for centuries, and must aid the child in his faith not deter it.

Play tapes all day long in the house, and let your children write out Surahs every day, say 5 ayahs a day or one small surah. One of the best pieces of advice I got when first learning the Arabic language is that “if you cannot read the Quran well you will never be proficient in the Arabic language”. There are many Arabs and Non-Arabs that speak, read, and write Arabic but their proficiency is blocked or stagnated because they simply have not mastered the best of Arabic ever spoken: The Quran.

This is all I can think off that this time. I am sure that other parents out there have more suggestions and tips. Also consult with ESL experts who can give you tips as to methods used for foreign languages in general.