But, in the anime, they have the baby pokemon, which evolve, not grow up. In the pokemon world, things do not grow up if they aren't human. It has to evolve at some point. Now, if it was just an aesthetic feature of Kangaskhan, I could let it slide, but it has been shown multiple times having been a separate entity, and that episode of pokemon I linked in the first episode shows it outside the mothers pouch, showing that they do not need their mother to live on their own. Also evidencing Cubone=Kanga. In the pokedex it says that they stay in the pouch until they're three years old, but that might just be because they aren't leveling up, because they aren't fighting. They might feed off of what the mother defeats, and slowly gain levels, sort of like an exp share, and then evolve. But if the mother dies, they have to fend for themselves, which would turn them into cubones.
I can see where you're coming from. Real-life joeys do go outside and hop around with their mother watching, but they remain with their parent until they can take care of themselves. When they are first born, they are extremely tiny and blind, so they practically live in the pouch for about nine months, feeding off the mother's milk. After that, they can poke their little heads out and move about, but they remain in the pouch for almost another year, and then they are free to leave. So it's very possible the baby Kangaskhan do just that.
But that still doesn't explain where the baby Kangaskhan would get the skull and bone club from. Clearly their mothers are two-three times their size, and it does take time for a body to decompose to the bone, and even then, Kangaskhan live in herds. If a mother dies, another can take care of the baby.