Is the meaning of "ten" in the hadith: "And beat them for it [prayer] when they are ten years old, and separate them in their beds" specifically the age of ten, or approximately that age? And is it obligatory to separate them in their beds if a girl reaches puberty before the age of ten, or if one of the siblings reaches ten and the other has not yet?

Children should be separated in their sleeping places when they reach ten years of age. This separation is recommended from the age of seven and may become obligatory if they are adults or if there is proximity of 'awrahs (private parts). This ruling is directed at guardians and applies to children who have reached the specified age; they should not share a single bed with those younger than this age.