When they were getting checks as little kids, those checks were going into their accounts. As they are getting older, they have a few grand to set forward with, but they also get to use the money.
When my youngest wanted to ride a pony for her birthday, she paid for part of the party - not because I have ideas about kids paying for their birthdays, but I was just really short on cash and let her know I was $XXX short for the party, and if she could pay for the difference we could arrange it.