Am I Overstepping?

We have a really good family friend who has 2 girls (7 & 10) that Sophie adores. He's my hubby's best friend, so they are more or less each other's brother. The girls play well together usually, but the last few times they've visited haven't ended so well. When they came to our old house I would notice the youngest daughter would spend lots of time in our bathroom & things were out of place when she came out. Toys that were not out in the toyroom would suddenly appear there as well as cupboards opened and items misplaced. Since we've been here, usually we'll play at a park or their house, but the last few times they have been here we've been experiencing the youngest daughter spend a lot of time in the bathroom with things out of place. Then I accidently overheard the oldest ask the youngest if she "had everything in her pocket" when they were getting to leave a few months ago. She had her coat akwardly zipped up and arms crossed--full of something. Luckily, she's not so smart, and tried to steal some snacks & a box of goldfish crackers of Sophia's so it rattled. We uncovered her arsenal of all she was trying to take & her dad didn't correct her or back us up when we discovered what she was trying to do.

Fast forward to Friday when hubby had to help his friend with car stuff, so the girls and I came along and I had to entertain them until his girls came home from school. We ended up in the playroom with the Barbies & Littlest Pet Shop animals when Sophie commented that they "have the same Barbie dress as me!". I looked at it and it was a very frilly, 90's style dress that I knew Sophie had one of (yes, from my personal collection! hehe). I marked a few of her toys with an "S" when we had a play date with a different friend, so I looked on this dress and sure enough there was an "S" on the tag where I had marked it. We also found a Barbie jacket with our specific "S" on it as well. When the guys came back in after a long while, hubby said it was time to go & our friend's ex wife was standing right there, so I didn't want to make a scene about what we found. We simply cleaned up the toys and took our two outfits back home. I showed hubby when we were at the store on our way home what we uncovered and he was very disappointed in the girls for stealing more of Sophie's things. Then yesterday they surprise visited us here and I felt like I had to watch both of his girls like criminals all day long. I cannot trust either of them after the stunts pulled, so when they were leaving, Sophie started wailing. After she calmed down (they were long gone by that point), she explained that one of his girls took something of hers in her hand (a toy? a piece of her left-over Easter candy?), so hubby and I agreed that we needed to tell his friend about this again.

I sent him a message explaining what happened and what we discovered on Friday at their house. I got a return message not too long afterwards saying he didn't find anything on her or in his car and that his daughter. His tone seemed to not believe me or take it really seriously that any of this happened, so hubby's going to discuss this with him tomorrow when they're doing more car stuff.

My question is: Was I wrong for bringing this up? Should I have just quietly taken back the clothes we found and not said anything more about it? Why do I feel like the bad guy when we're the ones being stolen from/taken advantage of and having our privacy violated by his daughters while he does nothing? I did tell him that if this keeps up, I will be forced to have to check the girls pockets/hands/bags when they leave to prove we can trust them enough to not steal again. This whole situation makes me livid--not only do you NOT steal, but especially not from your friends/family.

Sophie gladly shares her toys and goes out of her way to get the girls to play with her favorites. I'm so proud of how well she plays, so I don't want to discourage playtime, but I also don't want her to learn that it's ok to take things you want just because you want them. She did tell our friend's youngest daughter yesterday that she shouldn't take any of her toys home because it makes her sad & our friend just laughed it off.

I don't want hubby's friendship harmed by any of this, and maybe I'm just extra hormonal and protective so I could be overreacting, but I'm still very annoyed. I need to know I'm not in the wrong over here.

EDIT UPDATE: Hubby talked to his friend & basically his friend didn't care. His solution was to just not bring his kids to visit anymore (rather than stopping this behavior to begin with!). Great parenting there...or not...