You can bathe the girl with the boys if the girl is comfortable with it.
However if the girl is not comfortable being bathed with the boys then you should bathe her separately.
Also if the 10 year old has no health issues then she should be able to bathe or shower herself without any help.
Even the 8 year old and 6 year old should be able to bathe themselves unless they have any disabilities.
However with the 6 year old you sometimes have to ensure they actually bathe so sometimes they need to have help for that reason.
My own 6 year old son would not get cleaned up properly or sometimes would not even get into the shower or bath unless I was in the bathroom with him to make sure he got bathed.