Is it permissible for a girl who is nine years old to wear the hijab, despite the objection of some relatives who argue that it is still too early?

The girl should be encouraged to wear the hijab, even though she is not yet obligated to do so, because she is approaching the age of puberty. It was the custom of the predecessors to accustom their children to acts of worship so that they would become familiar with them. You are not committing a mistake by doing so; rather, you will be rewarded for it.