"The boy is free."
Generally used to mean the child is out of school or on vacation. Doesn't refer to price. I suppose you could also label the child as "free-range".
Possible but questionable - shouldn't really be selling kids...
The boy is free (able to roam around as he wishes): הילד חופשי
The boy is free (doesn't cost anything): הילד בחינם/חינמי