Sex play between 9 y.o. and 5 y.o?

Last night I was babysitting for relatives who have 9 year old boy who they adopted from China when he was 5.  He and my five year old daughter usually play together and we spend a lot of time with his family.  They were playing in a room together and I went to check on them.  The door was locked.  I knocked on the door and asked what was going on.  They didn't open the door immediately, and I heard scurrying and the boy tell my daughter to hide. I got the door opened and found my daughter naked in the closet.  I demanded to know what was going on and my daughter said "he made me do it."  The boy said " I don't want you to be mad at me, I made up a game, and she had to take off her clothes." I didn't know what to say or how to handle the situation at the time so I took my daughter and left the room.  I let her know she wasn't in trouble, asked her some open ended questions (not leading her, I wanted to know exactly what happened in her words) and she told me that they were wrestling and he pulled her clothes off, and that he also took his clothes off and that he touched her "down there".   Now, she doesn't seem traumatized by this and if it were done completely against her will I would have heard her protest as I was in the very next room on the other side of the door.  My queston is, is this normal behaviour for him seeing as how he is 4 years older than her, and how should I handle this? I am trying to make it not seem like a big deal to her, and have talked to her about inappropriate touching.  The boys mom thinks this is normal for him, and that he is on her level even though he is older because of the transistion of changing countries and languages.  They had a lot of behavioural problems with him when they first adopted him, but he has been fine lately.  We are very close and I don't want this to cause problems in our family relationship, but I also need my daughter to be safe.  Any thoughts would be appreciated.

[margypops]

I think you should speak to the parents of the boy again and no it isnt okay,the Mom is taking it too lightly ,as he may be doing this with others your daughter  is so much younger,I think you treated it correctly for your child  .Maybe make sure your daughter isnt in a room with a locked door and an older child , its always good to supervise them.