Missing Money

I have recently noticed money missing from my handbag. The first time I thought I had mislaid it or miscounted, but it has now happened a good few times and I think my 13 year old daughter is taking it, as I have noticed that she has bought a few things for herself. I asked her where she got the money from, but she just says it’s her Christmas or birthday money. I don’t believe her, but I don’t know how to take it any further, what should I do?