Online
Easy solution: Charge $20 (or more) per unaccompanied person under the appropriate age at the door.
If you bring receipts showing you purchased the amount of your "cover charge" or more from the time you entered, you get your "cover charge" money back.
Loitering is not a right and definitely should not be free.
They did this at a Houma bowling alley and it transformed the place. It was previously overrun with unattended children causing trouble while not spending money. Charging a $5 cover that could be applied to bowling fees really helped.