Apparently you still haven't realized that anything on the internet (such as information on how to RNG abuse) is freely available to all ages (despite age restrictions; it's not hard for someone to lie about their age). Searching "Pokemon RNG abuse" in google brings up Smogon's page as the first result. So really, it's incredibly easy to find the information, and with the ease of RNG abuse in Generation 5, it's not unreasonable for someone who is twelve years or younger to figure it out.
There is literally no way for them to tell the difference, and they won't ever be able to. All RNGed Pokemon are generated legitimately through the game, and are exactly the same as a random Pidove caught on Route 3 in terms of legitimacy.
There's a big difference between the two. Legitimate means it was generated by the game itself, whereas legal encompasses anything from the game itself, and anything created externally by a program such as Pokegen that could be possible in the game. One could even argue that due to the terrible Nintendo hack checkers, that anything that passes those is legal, such as Sejun's Follow Me Magmar he used recently (Despite being completely impossible to get such a spread for it).