Give them a finger and they'll take an arm...
For me it wouldn't be a problem. If I were paying my parent's allowance and heard that it's not enough, I'd give them a choice between keeping it as it is, or sending me all the bills. This would mean that I have to accept things they buy and pay for them, or not.
I find it hard to believe that a grown up would have none of their own money and had to ask me for every penny. I'm not paying for my parents, they have their own money, but if they asked me for help, I'd probably demand access to their bank account first, to see how they're spending money. Usually the problem lies in people's spending habits, not in a total lack of money. I don't know a single person who has 0 money, but I know a lot of gamblers and so called "big spenders" who have a room full of designer clothes, but can't pay the rent.