I also think the best way to do this is to have two installs of mamewah, one with ONLY the kids' game list, and another with all your lists. Have them be identical except for the games available, and have your version only launchable by a shortcut that's in your passworded user, or in the Administrator user, or just somewhere where you have to know where it is to find it. Your cab starting up into 'kid mode' Mamewah would be a start, then maybe you'd have to shut down mamewah and go somewhere deep into the Start menu to find the adult shortcut whereas the kidmode shortcut would be a big pretty icon on the desktop labeled "Arcade Games".
That said, I agree with HC here. You might be able to prevent your kids from playing the games by themselves, but how are you gonna keep them from walking in on you playing them, or keep from accidentally leaving the adult version on once, or keep the kids from coming home early from wherever you think you dropped them off for the evening and seeing daddy looking at their arcade game machine with [insert vivid description of really nasty hentai picture here] displaying on the screen?
If you have to keep adult games around, maybe the best thing is to take the adult games off the cab and only play them on your own computer, in your room... you can reliably keep them out of there, and a keyboard is better for mahjong games anyway right? Then, if your kids do figure out how to bypass the adult lock on the cab, all they'd be playing is Metal Slug, not porno, and you'd catch them at it pretty easily.
All I know is that someone I know is going through a divorce where the father keeps trying to win custody of the kids, and she's had her lawyer keep evidence of "access to porn at his house" because it would most likely result in her winning the case when/if he does take her to court.
(sorry if I'm bossing you around, I just don't want anything bad to happen to ya)