What is the best temperature for children who sleep and rest?

There is no specific room temperature that is necessary for babies & children to sleep & rest.
Red Nose has the following information about Room Temperature

It is not necessary to monitor the room temperature or to leave the heating or cooling on all night, as long as the baby is dressed appropriately for the room temperature.

Dress baby as you would dress yourself – comfortably warm, not hot or cold.

If using bedclothes rather than a sleeping bag, it is the best to use layers of lightweight blankets that can be added or removed easily according to the room temperature and which can be tucked underneath the mattress
https://rednose.org.au/article/room-temperature