Polite honorific term used to refer to someone else's child. Not used for one's own child.
How old is your child?
お子さんが描いた絵を見せていただきました。
I was shown the picture your child drew.
Mr. Tanaka's child is very lively.
How many children do you have?
What is your child's name?
Excuse me, but how old is your child?