Kangaskhan has a baby with her that looks a little like cubone but has grey skin. Cubone on the other hand has pancake colored skin. And since only a few pokemon can be encountered with different colors (wormadam for instance) we can assume that Cubone is in fact, not Kangaskhan's child. That's just color differences.
Adult Kangaskhan are pancake color just like Cubone. Cubone's color could be the result of maturity in that species of pokemon or just a side affect of when kangaskhan leaves it's mothers pouch.
hmm... true... true... But the fact that gamefreak/nintendo have yet to say that all are related still keeps me thinking they are not. Watch the last pokemon game come out and they say "yup, every theory of every pokemon you have ever made was true."
Marowak and child. http://fc03.deviantart.net/fs70/f/2013/189/4/0/marowak_and_cubone_base_by_riocari48-d6cmhxd.png
Kangaskhan is darker than marowak. There are theory's that Kangaskhan's child evolves into cubone which evolves into Marowak which evolves into Khangaskhan who dies and then the baby takes her skull and the cycle repeats.