I think setting spending limits well ahead of time is a smart move. It gives a teen some freedom, but it also teaches that everyone must act within limits -- even on vacation.
I can see checking the balance every day (especially with a teen-aged charger), but why do you want to pay off the balances every day? It won't save any money, and it'll take your time.
As for children's charging privledges, someone here said that you can buy a pre-paid VISA card and write that card # on your son's boarding documents (and your own info on your own documents). Each of you will have a charging info card along with your documents, and you can give different credit cards for each person in the room. I'm not sure whether you could "mix" this method with your cash-only idea. Disclaimer: I have not used this pre-paid VISA method personally, but it sounded good to me.
How much is enough? I didn't think there was much in the way of souveneirs onboard. We didn't buy much at all. If you're going to let him get the soda package, that'll be $35 upfront (which will blow his $25 right away on the first day -- perhaps it'd be better to say X amount for the whole cruise). My kids didn't do the arcade (there are free video games in the kids' clubs -- don't know about the teen club); does he have a Gameboy or something similar? He isn't going to buy alcohol, he isn't going to have his hair braided, and he isn't going to go to the spa. I think $25 per day is gracious plenty -- even extravagant -- for a boy this age.
Warning: If he does any excursions (even if YOU reserve them along with the rest of the family), they will show up on HIS bill. For example, my daughter didn't buy a single thing on the ship, but she had a charge of almost $100 for the dolphin excursion, another for $14 for the bananna boat ride, and another for $6 for rental of a float on
Castaway Cay. So read the details before you become angry with him!
Finally, if he does happen to go overboard with purchases, you can have guest services "punch" his card. It'll then have no buying power. If he's a responsible kid, just knowing that his can happen will be enough to deter him.
Finally, I LOVE the idea of "matching" a child's savings. I will definitely use this one with my two girls.