Some money gets held out of their pay checks to cover their expenses. Their pay is automatically deposited into their federal account. There's a bank on base with an ATM and they can withdraw money as needed. NAPS education is free, as is room and board. Their pay more than covers their expenses, but not by a whole lot.
Some parents pay the uniform and books costs and let the kids live on their pay.
Some suppliment the pay checks and let the kids pay expenses. Many kids are on their own. How folks work this out is up to families and their own financial situations. Do what you are comfortable doing. There is parking.