When a 70 lbs dog is bitting a child...this is a very serious situation.

First off.. you should NOT tie the dog to the child....
someone is going to get hurt...most likely the child.

This is potentially very dangerous advice. I repeat please do not tie this dog to the child. Do not do this.

You need to contact your vet, tell them the problem and ask for a referral to an animal behaviourist. This is a serious situation and you need professional help from those trained in this area.

Until someone is called in to assess the dog and situation, explain to the family what is going on, offer their opinions on the risks of contiuned interaction of this dog and the child and train the family how to deal with this dog...
.... the dog should not be allowed loose in the same area as the child is in. Another words put the dog in the back yard and keep the child out of the yard.

This child has a legal right to be protected from this dog.

Deb Kelly