I've made a similar deal with my kids: Around 7 years ago I set up a "kid retirement" plan for them, where they couldn't touch the money until they were 18, but any money they put in I would match, and I'd also give them 10% APY with monthly compounding. My daughter aged out of it a couple years ago, she got something in the $100 range. Her brother still has a 18 months left, and I just recently rolled his over into the custodial account, he's got over a grand in there currently.
My daughter I just recently set up a ROTH for her and told her I'd match anything she puts into it, and stressed she should put something into it now from her savings, and then put some of her paycheck into it, anything is better than nothing. So far she's declined the free money. I'm going to set one up for my son, once he's at the point of having an income to justify it. She's very smart, but in some ways she's very stupid.