It is indeed a whole separate pokémon—a baby kangaskhan, in fact. Now, I know that there are certain simulations out there (some may even call it a “video game”) in which players who breed kangaskhan end up hatching adult-baby kangaskhan pairs straight from a single egg. I’d like to tell you that these simulations are, in fact, ridiculous. Basically, what actually happens is that an adult kangaskhan carries an egg within its pouch that eventually hatches into a joey. The joey remains in her mother’s pouch (I say “her,” as kangaskhan is an exclusively female species) for the next three years until she reaches maturity and develops her own pouch. After that point, she leaves and finds her own mate. In tamed situations, this means the mother of your kangaskhan egg will do the same. If you’ve obtained your egg from a breeder, then that means you’ll end up with a joey, and it’s important to keep that joey warm for the next three years (usually within an artificial pouch that you strap to yourself).
As for whether or not it’s safe for a kangaskhan to battle while carrying a joey, it depends on how old the joey is. Extremely young joeys (those a year old or younger) should be kept away from the battlefield, but their mothers will often refuse to battle, making this not as much of a challenge as it sounds. Older joeys are typically stronger and more resilient, and as such, their mother will begin to seek opportunities to battle so she can teach them how to do it on their own. She will usually go to great lengths to keep her joey safe, so on your end, it is considered safe because the mother will never enter any situation she thinks would put her young in danger. In fact, in some cases (such as in “mega evolution,” which is in quotes because of things I’ve mentioned in previous posts), the mother will push her joey to battle alongside her.
In other words, is it safe? For younger joeys, probably not. For older joeys, it’s not only safe; it’s also highly encouraged. Of course, either way, the mother kangaskhan will let you know fully well when her joey can battle and how intense of a battle she may have. It would be a good idea to listen to the mother kangaskhan, of course, as the mother kangaskhan may know Sucker Punch and definitely has fewer qualms about attacking her trainer than most other pokémon.
The Pokédex According to Bill 