It depends on your purpose of the gift - is it because you just want to give them something or is it to "remember" your son's baptism? If it is for a keepsake, then something like a Bible or a cross to put in their room at home is nice. However, it is a "thank you for coming" or "it is good to see you" gift then something more age appropriate and of current interest for each of them might be in order (if you don't see them often then check with the parents if Diego or Thomas the Train are their current LOVES). Someone suggested something to "keep them quiet during the baptism" - I recently got my daughter a Dora the Explorer book and magnet play set which was a big hit for a recent LONG plane trip (search Amazon for "Discover with Dora book & magnet playset) - at the bookstore where I got mine they also had Sponge Bob, Spiderman, and some others.
Good luck and many blessings with your new baby and his introduction to his Christian family.
peace,
Stacy