I would say a neck strap isn't a bad idea, though as others have said, prescription glasses are usually pretty well fit, and don't tend to fall of so easily, still, the neck strap is a cheap bit of insurance.
The place that we had the most trouble with eye glasses when my son was young was at 3D shows. He was never comfortable wearing the 3D glasses over his prescription glasses, so he always took off the prescription glasses, and was not great about handing them to his mother or I, because of course, he could handle it on his own. Inevitably, they got left behind in the theater. Thought Disney is pretty good at recovering such things, we did wind up having to replace glasses twice before he outgrew that.