What's wrong with the following? stop. the glass is about to break above the kid's heads

"Heads" indicates that "kids" must be plural, but with the way the apostrophe is placed, it is singular. Instead of "kid's," it should be "kids'." Or the other way around – "head," instead of "heads."

The “kid’s heads” implies that there is more than one child there whereas an apostrophe for “kid’s head” shows possession of a kid’s head.