As you know, there are no specific formulas for human behaviour. In some cultures, the children sleep separately from the very beginning. Sometimes in a crib in the same room, quite often in another room, with a small mike system to let them know when the baby cries. It seems to me that since your daughter is 6 years old, she is certainly ready to have a bed of her own. If you have the facility, this may be a good time to shift them to their own room with two beds or at least to their own separate beds. Stay with them as they fall asleep, read a story or sing to them. They will enjoy having a space of their own. Do this as a privilege for them. Once in a while, they may want to cuddle up and sleep with you, especially if they are sick or scared. You could permit that of course. Children like to be independent.