If it states that it's for donations, and you use it for something else, you could be committing a felony. At the very least, you'd be breaking the terms of service of both YouTube and PayPal. If PayPal catches stuff like that, they have the legal right (and most often enforce it) to re-take all the money done through those transactions. YouTube would likely put a penalty on the channel.
My point? Tread carefully whenever actual money is involved. Keep in mind, that the terms of service you agree to on YouTube and PayPal, are legal binding contracts - breaking them, if severe enough, is an actual crime.
Not telling you what to do, just be sure you know what you're doing.
Instead of setting up a donation link, why not sell merchandise that's actually useful to people? It's not like you'll be raking in mad money with this donation link anyway. If WikiPedia can barely get enough donations to keep their site running, well... yeah.