Games With 10-Year-Olds

My granddaughter won money in a music composition contest and wanted to spend $4 of it on some artificial fingernails.? I accompanied her to the store; being a proud grandfather, I’d like to reward her for winning the contest.? So after she picked out the nails and we walked to the cashier, I offered to pay half.
I figure this way I am transferring income, showing pride, but not lowering the price of fake nails and not giving her an?incentive to spend more on fancier junky fake nails. This little?strategy seems sensible for a one-shot game; but in a?repeated game I know that she will catch on and spend more next time, expecting a subsidy from me.??I like to transfer income, but I don’t want to subsidize specific purchases. I need to cook up a new strategy for the next similar situation.