When they reach the age of 10, their bed should be separated but if you feel the necessity before, you should do it earlier.
هكذا فى الفتاوى رد المحتار ، ص:5/ ج:215 ، (باب الاستبراء وغيره من كتاب الكراهية).
There is no difference between the age of boy and girl according to ruling, but however the parents should consider the necessity for the same. If they had any fear, it is alright to make their bed separate.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband