4-5 months are abolsutely safe. At the age of 6 months or older, parents start chasing around offspring of their own sex and latest then, they should be removed.
In some cases, offspring can stay in ther parents enclosure for almost ever without any sign of aggression. This however, in most cases indicates that the enclosure is too small and the seemingly peace is just a consequence of a crowding effect.
Even halfgrown tokays do not harm their youngest siblings. The only challenge for the keeper thus is to provide enough food within all size classes needed for a growing family of tokays. Key is, to feed a lot of tiny insects, because even the larger specimens prey on them and if small feeder insects are not given in excess, hatchlings might not be able to catch enough of them before they are eaten by the rest of the family.
Keeoping hatchlings in pairs in fact is not a good idea unless the enclosure is larger than 20g from day 1 on. Raising in groups works much better. I typically raise all babies of one pair from one year in the same enclosure (50g and up), if they do not stay with the parents.
So far, survival rate during the last 10 or more years is 100% and all grow up nicely.
best
Ingo