You cannot apply funds to a Magic Band but you can add them to the account to which the bands are tied. We always charge everything to the room on our stay so I take my rewards card to the front desk and ask them to apply the entire card balance to my account. That way I use it up in one transaction and don't have to keep track of the rewards card balance as I use it.
You can also apply the gift card balances to the room account but if you give your kids charging privileges I don't know if you can set up a spending limit on their bands.