Here is a video that shows 90 seconds that could save your life. How to actually MAKE A FIRE with a lens, rather than just burning a hole in a leaf. (Or frying ants, which seems to be the other thing that kids like to do with magnifying glasses.)
By forming your target material into an efficient ball, you will be able to start a fire even with very small lenses. Like less than an inch across small. The finer the individual fibres, and the more densely they are packed, the more effective is your ball of smigtin (smouldering iginition tinder).