I wouldn't give a kid under say, 10 or 11 a light that puts out more than about 50 lumens. Even if he or she isn't going to look right at the emitter, playing around with them, he's gonna shine it in someone's eyes.
Time was 60 lumens was considered adequate brightness for a "tactical" light, meaning it gave you enough light to identify a threat at across-the-room distance, and enough light to male someone in total darkness turn away from the beam if it were directed into the eyes. At least, years ago, that how I understood it. I've pretty much kept that as a rule of thumb: if its 60 lumens or more, be careful where you point it.
At what level will a light cause eye damage? I'm not sure anyone is really sure; probably not any controlled studies on this. I'd operate under the assumption that you don't want to shine anything in your eyes, or your kid's, or your kid's friends, that's brigher than what the doc uses to check your eyes in an exam.