I am looking for my 2 yr. old daughter. also!
I just looked online at Gap and missed their call for ad models but you can keep checking their website. I went to Wilamina or Ford (in NY) but they do not represent young children. I plan on looking at other top agencies in NY to check to see if they have regional auditions/offices. I also suggest looking into magazines such as Parents and Parenting for ad work.
I haven't but I know some people that have. Through what I have seen/heard, my only recommendation is to go through a GOOD talent agency. Do your research and get some good headshots and professional photographs of your child and start building a portfolio. I think that no matter how beautiful or cute the child may be, your child should have a natural talent and enjoy it. The last thing you want to do is force your child and make them unhappy! Just don't be a pageant mom like the ones on TV!