For an icon that generic, it's going to be hard for anyone to say what it is definitively. I would really be best if you check your child's phone periodically to see if it's there again, and when you see it, just borrow the phone briefly and swipe down the notification panel to see what it is.
If you're wondering why we're being so cagey about this, it's because this kind of question is relatively common on this forum, and not infrequently it's from a person who's asking about someone else's phone without their permission (i.e., they're snooping). Obviously, a parent should be able to monitor their kid, but since we can never be 100% certain of the relationship being presented, this is why we're always a little careful about these kinds of questions.