You cannot. It has to be done using the account which is doing the subscribing. At this stage it really becomes more a parenting issue, I'm sorry.
One option would be to go ahead and pay for these subscriptions this time but then turn off (or threaten to turn off) purchase sharing. From: Add a family member to your shared subscriptions - Apple Support
"When Purchase Sharing is turned off, everyone must use their own payment method. When Purchase Sharing is turned off, family members can share subscriptions, but they can't share individual purchases like apps or books."
Since the children probably don't have their own cards, and Apple would require a card for a subscription, this would effectively prevent them from making purchases except for things where personal account balance might be accepted.
Here are some features that may help against future issues:
Use Screen Time as part of parental controls on your child's iPhone, iPad, and iPod touch —> Use parental controls to manage your child's iPhone or iPad - Apple Support
Consider activating Ask To Buy—> Approve what kids buy with Ask to Buy - Apple Support