Before donating you would need to come to an agreement about the child knowning who you were and at what age. Also to trust them to hold to that agreement.
More importantly, I think unless you go via a government registered fertitlity agency ie a fully legal procedure, you remain responsible for child support no matter what the informal or written arrangement with the mother was. It will be out of her hands and into the dreaded Child Support Agency.
So, not good for you if it turns out to be twins who want to go on to higher education… 20 years or so of double support payments. This sounds a joke, but its a serious consideration. If you end up having to pay child support you might find that your and your own future partner not only struggle to buy a house etc but cant afford to have any children of your own as a couple.
Remember how ruthless formerly decent people get over money in wills? It will be nothing in comparison to how they fight to 'get the best' for their child – if that means ruining you and your future to help their child, chances are they will take the money and run, as they will love the child more than they love you as a friend.