What is the best sleeping bag for a child?

How old is the child. I would not let children under the age of 5 in a proper sleeping bag- perhaps a sleep sack (they are different things).
A sleep sack which is only slightly longer than their shoulders when they are standing up, and then a separate pillow for the head.

If you are using a sleep sack (which is not intended to be used out of doors, more to put the child up for the night on say a sofa in the living room but no blankets to worry about and the child feels secure). Some of these sleep sacks have little pockets where you can tuck an extra blanket over the top if it gets cold. Always keep and eye on the child even if it is only once in the night say half way through their usual sleep pattern.
If the child is much older say 8 and over a proper mummy sleeping bag which is lagged and has an inbuilt pillow and head cover so that they cannot get cold (I assume they are sleeping outside),
If they are sleeping outside, make sure they are sleeping on a ground sheet (large, deep, plastic sheet) to keep any damp rising from the ground, particularly if it has rained recently, or there is a lot of dew forming- especially spring time. That way the damp cannot get in to the lagging of the sleeping bag and make it damp, which then defeats the object of the bag, since the body is then trying to dry out the bag. Quickest way to get cold.
If they are sleeping in your back garden, always make sure the back door is not locked shut so if they do change their mind they can get in to the house and get in to their bed. Keep a low level light on for them. The younger the child, the more important this “escape route”.
If they are sleeping away from home make sure the sleeping bag is adequate for their needs, and again they have a ground sheet if they are sleeping in say woodland.
Finally don’t buy the sleeping bag for them without them there, let them choose the bag they want. Then check it over to make sure that it is going to be satisfactory and if not explain to them why. Then buy it.

Article from https://letcase.blogspot.com/2019/02/what-is-best-sleeping-bag-for-child.html