The humble beginning of what would become Capcom's highly successful "Vs." series of cross-universe fighting games--but Children of the Atom has just X-Men hero and villain characters. The action is slow-moving by the standards of the later games in the series, and plays pretty much line an ordinary 1-on-1 fighter, with some special moves and a single-charge super meter.
I seem to recall I had trouble getting this game to run on my PSOne, but I could be wrong about that. No problems with it on a PS2, with or without the fast Disc Speed setting.
Well I was totally wrong about COTA having "just X-Men hero and villain characters," because Super Street Fighter II Turbo's Akuma is present as a hidden character--a conditional mid-boss, apparently, but also playable via a button code. GameFAQs says the code is
"For Player 1- at the character select, quickly go to Spiral and leave the cursor on her picture for 3 seconds, then press Left, Left, Left, Left, Right, Up, Right, Right, Down. Count another 3 seconds and then press L1+R1+X. Silver Samurai's picture should disappear and Akuma will drop from the top of the screen."
but those buttons are only accurate if you haven't remapped your controls; properly speaking, the buttons to hit are LK+HP+HK.