I would obviously give someone a good beating, but I wouldn't kill them. What good are you to your kids when you're in prison for their whole childhood?
Sound's like a case of a looney being let out of the asylum too soon to me. Any ****er who stabs a 4 yr old is definately a Looney of the highest order, no matter what the circumstances. Rehab in the community bollocks, keep em' locked in so they don't do any harm to others.