My step son is stealing from my house?

My partner and I have a 3 month old together. I also have a 16 year old and seven year old to my husband who passed away. My current partner has a 12 year old daughter to one woman, and a 11 year old son and 7 year old daughter to another woman. He has his kids over every second weekend and any other time the mothers ask him to, which lately has been most weekends. These kids are very poorly behaved. 2 visits ago the 11 year old boy was standing on the edge of the pool peeing in it and the 7 year old poohed beside the pool. Neither child was smacked, just told if they did it again they would be banned for a while. They run around my house all weekend. Make mess everywhere. Grab fresh towels for every swim and every shower. Only eat crap food. To the point where I really can't be bothered cooking for them any more cause they just say Yuk! and don't eat it. The very last visit they were going through my boxes downstairs, these boxes where sealed. I caught them and told them in no uncertain terms to stay out of them. I have my dead husband in an urn in one of those boxes and a heap of other valuables. I also have a presents box with new stuff I had bought on sale that is great for gifts. This box had been gone through. Again I said stay away from my things. The night after they left, I found tags from the new things in the box discarded on the floor and noticed that the items attached to these tags had disappeared. I told my partner and he rang his son and he had the items. He said my 7 year old daughter said he could have them, which I know is rubbish because she was the one that alerted me to them going through my stuff. My partner saw he had some of his stuff from downstairs when he was leaving but decided he could have what he had found of his. He asked him what else he had in his bag, and he said nothing. I should have insisted we check his bag but didn't. My partner believes that he hasn't done anything wrong because he believes my 7 year old said he could have my stuff. Forget that I said said stay away from my boxes and even asked him to reinforce that. I get so frustrated. These aren't my children, but this is my house. Shouldn't I have more control in my own home?