First, you did a good thing, there was no comeback, and so it's all good. But I want to answer your more general question for other people in that situation,
Many buffets have and explicit 'no leftovers no sharing' policy, if they don't make it explicit up front then they undoubtedly will if you start abusing the buffet. So while sneaking someone a few chicken wings (or whatever) may go unnoticed it becomes a slippery slope to feeding a ton of other people for free. And, if you're caught, they will attempt to make you pay. If you get caught the best case is you pay for a buffet for an extra person, worst case -- if the buffet is included in the room rate -- you pay for an extra room.
While I would have done what you did I would probably recommend other people to offer a little money to the towel boy, or offer to take him for breakfast somewhere else. The chances are the hotel knows about this and are OK with it, but it's also possible that it's a setup for a scam which is why it might be better to offer something else.
But you should never feel unable to say no if it's something you're not comfortable with. I don't want to sound boorish but it's OK not to give if you don't want to. I don't imagine you're the first person the boy has hit up for food, nor will you be the last. Similarly I'd wager that he's pretty used to being refused and it's not going to affect him. Sure it might make you feel bad, but then I suspect that's the point.
You could try and address the larger problem (that the hotel employees? or hangers-on don't make enough) either via the hotel website or other review sites. But that's not really what you asked.
In short, if you want to help, offer some help in some other way, because sliding stuff off the buffet has potential to get you in a heap of trouble. But, if you don't want to help, don't be afraid to say no.