If all of their friends have a PS3, then the no-brainer answer is get them that. If all my friends had a PS3, and I got a wii, I would have been pissed.
Also then they will actually be experiencing games that may not necessarily be targeted towards, and who says thats a bad thing..
Another thing is that in my opinion, the PS3 is more future proof for when these kids grow older, and it is more of a family entertainment center than just a game console. I don't know if the family already has a blu-ray player, but if not, now they would.
Overall, I dont think the gap in number of good family games on the ps3 vs wii is large enough to justify getting a wii over a ps3, from the stuff I have seen from experience, most of the stuff is shovelware garbage