My cousin registered her kids with AdKids last year. She paid $630 for her two kids to be "represented" for their entire lives. She has to take them to one of several AdKids approved photographers every 6 months to have their headshots updated. (She has the option of purchasing photos from the photographer, if she wants. But the photographer will send ALL the photos he/she takes to AdKids and AdKids decides which one will be used as their updated headshot. FYI: My cousin said she thinks that they always pick the WORST photo of her kids.)
AdKids is not an agency - they work with agencies. So, for example, an agency would contact AdKids and say "Get me headshots of all of your red-haired kids between ages 2 and 5." AdKids would provide the headshots and the agency would then decide which of those kids they want to come in for an "audition".
In the year that my cousin has had her kids represented by AdKids, she has received only one call for her son. The "audutions" are usually during the week so you have to either be a stay at home mom, or have a job where you can take off at the last minute to take your child to an "audition" he/she may or may not end up getting.
As far as modeling goes, it's probably one of the best deals out there. Other places require you to pay much more PER CHILD, and there can be annual fees involved as well. AdKids charges one rate for ALL of your children, and there is nothing more to pay, ever. But you may not get many calls, and if you do, you have to be available at the drop of a hat to take your child to the call.