Because it will point him/her to the cross.
I just read this article from Desiring God’s blog. It’s a really good explanation of why teaching obedience and godliness to young children does matter. Read it – it’s worth your time.
The child will have a hard time grasping the meaning of the cross if parents have not required of him behaviors, some of which he dislikes, and none of which he can do perfectly.
Christ lived and died to provide for us the righteousness we need (but cannot perform) and to endure for us the punishment we deserve (but cannot endure). If parents do not require external righteousness and apply measures of punishment, the categories of the cross will be difficult for a child to grasp.