Possibility #1: The 'baby Kangaskhan' is actually a smaller twin.
Possibility #2: The parent is so overly protective that the babies are literally unable to fight until they're adults themselves. meaning mother kangaskhan technically gets partial dna cloned into baby to protect its own baby.
Possibility #3: The game designers don't feel like making a Baby Kangaskhan Pokemon since it has been shown for so long in its parent's pouch that seperating them would require extra work.
Possibility #4: Too much energy put into musing about such a minute detail of a massive franchise will only lead to headaches and Wild Mass Guessing.
Game FREaK LaziNess!!! 10/10 for that last comment in my opinion!