grammatical number - "On their back" or "on their backs"?

After the therapy, eight children (43%) became able to crawl/move on their back.

Or should I use "on their backs"?

Singular because each child only has one back, or plural because we're dealing with eight backs?

Answer

Plural, because we are, in fact, dealing with eight little backs. And a back is a back, no matter how small. A child has a back, but children have backs.

I'm sure others will back me on this.