With all due respect to the previously accepted sophisticated answer, which is correct as far as the original English language is concerned but incorrect as the answer to this specific question, I strongly believe that the average 10-years-old has absolutely no idea what any of that is, in the answer, and is using the word in an evolved manner that has nothing to do with the original English language.
A 10-year-old, and even teenagers nowadays, most commonly use the word “meta” to describe something that's awesome or ... “uber”, which is perhaps an outdated synonym, or even “OP”, which is short for overpowered. OP, and even the word “broken”, originally referred to game skills/spells, game characters/heroes, which the gaming community believed to be incorrectly programmed (“broken”) in a way that made them overpowered, or so good that they imbalanced the game. However, as games started to become more and more actually balanced, and the gaming community also realized that they are as balanced as possible, the terms OP and broken weren't that accepted anymore. A new term had to be invented. Meta is that word.
Meta, for kids and teenagers, and even for adults involved in the gaming scene (video and computer games), is an acronym for “Most Effective Tactic Available”, and the term started in games and the gaming scene to refer to the best things, tactics, skills, or characters to use to win the game. However, as it became more and more commonly used, it moved from the gaming community to young people generally, and it is sometimes now used instead of “awesome” or “uber”. As desirable as it is to think that our kids and teenagers are using “self-referential” terms, I really believe that this is actually your right answer. Check it with your kid and many other kids. Meta is the new awesome.
In short, when a young person says “meta”, they mean “awesome”. And the term is an acronym for “Most Effective Tactic Available”.