Hmm, I believe this belongs lingering in the 'Game Help' threads, though that's merely my personal opinion since you need help involving in-game.
Anyways, of course you can be a cub. Anyone can be a cub via in-game. All you really need to do is lower your Height bar from the default and make your lion/lioness resemble a cub. Tis' simple.
Normally individuals lower the Height bar either all the way down or near to the lowest setting. That's considered a pup/cub size in-game wise.