It's certainly possible there could be all manner of methods present to prevent kids from ever entering the game. Although at the same time we are talking about a game that purposefully locked players in and at least claimed they will die in RL if they die in the game. So, I wouldn't really trust any system meant to keep children out in this particular case. On the other hand, if we assume there was some effort to prevent kids from ever acquiring the game even if there was nothing to stop them from entering, then it does explain their lower numbers, but it absolutely contradicts with what we saw in this episode: if they had to be crafty enoug to get the game, they definitely shouldn't be the losers we saw here. They should be either HC or wannabe HC gamers already, despite their young age. Surre, there might be an occassional spoiled brat... But all of them? No way.
Asuna being surprised amounts to nothing. She's a part of the same poor writing, after all.