11-year-old tying up a 6-year-old

We brought them over on an early Friday, and the girls slept together in my son's room, who was gone for the weekend. That was no problem.

Saturday, about 1100, we went to the store, and asked the girls if they wanted to come along. They said no. We told them we would be back in a few hours. I didn't know much about GF, but figured it would be OK. We lived in a quiet neighborhood.

When we got back we didn't find the girls anywhere outside, so we look around the house. We keep our bedroom door closed, and looked there. Nothing. The basement. There we found GD stark naked, tied by the wrists hanging from a large ceiling beam, her ankles tied together and tied to a heavy iron pipe on the floor. Her mouth and eyes were covered with tape. GF was also stark naked, playing with our GD.

I yelled out, "HEY!" I never saw a little girl jump so hight as GF did. "What are you doing here?"

"I just had her tied up like we do a lot at my dad's house." She stood there just looking at me, not trying to cover herself up, as if she wanted me to see her naked. I immediately untied my GD and removed the tape, and she told me, "Grandpa, we weren't done yet!"

I told them to go get dressed. Nothing else was said and they played together as if nothing happened.I told my D&SIL about what happened, and they said they would take care of it. But now, I feel really angry, really betrayed, and a whold pletheora of emotions.

I don't think I could have done anything else though. Does anyone agree? Or could I have handled it all differently?