A new box of crayons and a thick pad of paper ... Write each child's name on the cover of his or her sketchpad.

Some Hotwheels cars (good for either gender)

A 6-pack of Playdough - as long as the parents are okay with that

Girls - hair stuff - a bunch of different colored pieces of ribbon, or if you want to spend time, get some ribbon and tie a bow onto each ponytail holder or barrette ...

Books they might enjoy are always a good, long-lasting gift, one you can read to them at bed-time next time you sit for them ...

Get some inexpensive t-shirts that are a bit big for them, and some fabric markers, customize for each kid.