Since they're up, playing and using the litterbox, I'd just keep an eye on them. Prozac has been prescribed for cats in the past. I remember one time my cat chewed a Effexor capsule! I really panicked because I know what Effexor did to me (put me in a drunken stupor). I called the vet and he told me that since the capsule was only chewed a little, chances are he'd be okay. Turned out he was okay.

I'm just glad to hear that your kitties are playing and aren't suffering any kind of bad side effects.