Apple Pay ?
I know that you can activate. “asked to buy“ for your children’s apps/itunes. Is there also a way to activate ask to buy for your children to use Apple Pay/credit card loaded onto their phone?
iPhone XR
Have you setup a Family Sharing Account, if so an Apple Cash Family may be what you’re looking for. Please review the Apple Support articles I linked below.
Set up Family Sharing - Apple Support
Set up and use Apple Cash Family - Apple Support