Good idea. I match my HS and college kids Roth, and pay a 6% dividend in the "bank of Dad" savings account they have. This is just a checkbook register that they deposit babysitting, gifts, etc. funds in. The actual funds go into my account. I pay the dividend on 1/1 every year.
Be careful not to penalize a saver. If child 1 saves 10% and you gift 5%, great. But if child 2 saves 15% would you gift 0%?
I'm a fan of equal gifting to avoid bad feelings. Also, you need to balance are you making a gift, or are you trying to control them? Sounds like a gift to me, but how will they (and spouses) see it?