My boyfriend's daughter steals from me and lies about it! It's kept me from allowing her to come visit.?

My boyfriend has a few children from previous relationships who all live with their mothers. The three youngest ones are from the same mother; two boys (7 and 9 years of age) and a girl (11 years old). About 5 months ago, the three of them came to visit. In preparation, I had gone and purchased a few crafts for all of us to enjoy during their stay. I put in extra thought and money into purchasing beads for my boyfriend's daughter and I to make matching jewelry out of. The boys were doing well with their poster coloring and my boyfriend's daughter and I were happily beading away. We both finished our projects (beach sandals) but had a few beads left, so we decided to make little extras out of the extras; she made a matching bracelet for herself and I made a collar for my dog. After we were finished, I hung my dog collar on a hook on the wall, but then she asked to see it. I got it down and handed it to her and she wrapped it around her feet like a double anklet and it fit beautifully. I forgot about it and we continued the rest of our evening by watching a movie. Before she left, she made sure she gathered all her jewelry and her father helped her put them on her teddy bear, which we had also purchased for her. We then took them to their mother's home. When my boyfriend and I got back, I realized my dog collar was missing. I searched everywhere before asking/telling my boyfriend aobut it. He immediately insisted that I must have misplaced it, but I didn't. I insisted that hadn't and requested that he call her and simply ask if she had "accidentally" taken it. He huffed and puffed, but gave in. She denied it, saying she put if back on the hook. She can't even reach the hook. Since then, she's been back twice, and I'm missing a pair of very expensive earrings my mother had given me. (VERY sentimental to me.) I haven't told my boyfriend about this. Because of this, I won't allow them to visit us at home anymore, but it isn't fair. Especially to the boys. HELP!!!