I did that all the time as a kid too! Funny that I don’t think of it much, because now I work in a oddly similar field - melting glass with a torch.

A magnifying glass produces a small point of heat, with fairly low temperature and area. Many people in history have worked glass with no eye protection without serious consequences, other than cataracts perhaps (many older Italians still do not wear didymium glasses, much less the ones with welding shades).