The lab may be a good choice. I think a lab or retreiver would be your best bet. Herding dogs would be great for it... but like mentioned, sometimes they may nip at heals.
No matter what your decision is with the breed of dog, you will need to do extensive training with him/her.

It could be done thats for sure.

You're best bet is to research to see if it has been done before & how they trained. Are you a member to down syndrome sites? Maybe they have more info on service dogs for children with down syndrome?

I wish you luck.