A few relatives from my side told us the man shouldn’t be changing the daughters diaper as they say it’s inappropriate and now we’re conflicted. Is there any Islamic justification for this to be inappropriate/ wrong of the father to do?

There is absolutely nothing wrong with your husband changing his daughter’s diaper. This is purely cultural. In some cultures, it’s looked down upon for a man to change his kid’s diapers. It’s seen as a task that only a woman should do. But Islamically there is nothing wrong with it. It’s completely appropriate.