The creator clearly was wicked to allow children at all.
After all, it seems pretty clear it's possible to prevent children from playing the game.
Nervgear being able to digitize the face appearance, interacting with brainwaves and also the calibration (Klein/Kirito explanation)
With those 3 key sources of data, you can clearly exclude children, it might be a little more difficult for people around majority (16 to 20) but under 10 or even 15 should be fairly easy.

So that guy then locks everyone in a deathgame, without even taking children into account.

Gives another angle to judge the creator of the game.