The 7-year-old boy -- Legos. My son and all of his friends (all seven years old) love Legos. Another idea is a skateboard & helmet.
The youngest girl -- A Pillow Pet. My daughter is four and a girly-girl and this is on her wish list. I know other parents of girls this age and this is also on their wish lists. Get one now. It made the "Hot Toys" list this year and it's starting to get sold out.
Can't help with the middle girl because I'm not experienced with that age group with girls yet.