M.M.
Cash is always good. Traditionally, in sums of 18. For instance you might give $36 to each. Chai is is the symbol of life and the two letters in the Hebrew alphabet that comprise the word total 18. The Het amounts to 8 and the yud has amounts to 10. You could also give something that interests the children. Maybe they have a charity their family supports?
Anything you might give for a birthday present, would also be appropriate.
Back in the day, I remember receiving cash, savings bonds, donations to a charity in my name, and someone planted trees in Israel in my name. I also got a popcorn maker which I took to college with me eventually.