If it actually says "posthumous baptism", then it will mean that one or both of the parents died before the child was baptised. Quite often, though, it will say "posthumous child of", which means that the child was born after the father had died, and possibly in very rare cases the mother. It could be recorded as such for the sake of inheritance or propriety or possibly both.
I don't think children were ever baptised once they had died, or at least, I've never seen any such record.
Chris