If they're talking about a specific child, sure, but if they are talking about a single, unspecified child, they is becoming more popular[1].
"Bill is over there. He is playing with his toys."
"If your child is in the kitchen, make sure they don't touch the hot stove." vs "If your child is in the kitchen, make sure he/she doesn't touch the hot stove."
For many readers the he/she breaks all flow the sentence had.
"Bill is over there. He is playing with his toys."
"If your child is in the kitchen, make sure they don't touch the hot stove." vs "If your child is in the kitchen, make sure he/she doesn't touch the hot stove."
For many readers the he/she breaks all flow the sentence had.
[1]: http://en.wikipedia.org/wiki/Singular_they
EDIT: s~he/she don't~he/she doesn't~