Ok, I'm not even going to go the 'sleepwalking' route here, because that just seems so over the top (even for this forum) and OP said there have been several other sleepovers with no incidents or other sleepwalking tendencies.
Either one or more of the kids had a bit of fun and don't want to own up to it. It's what kids do. Thankfully, no one or nothing was seriously harmed.
When I was around 10, I was at my aunts house, and I found her contact lens case in the bathroom. I, of course, knew better than to play with it, but I was so intrigued I opened the case and one fell out. When my aunt came later and was looking for them, I knew I would get in trouble, so I lied and said I didn't know what happened to them. It was obviously me.... as the only other people in the house were my teenage cousin and the cat, but I steadfastly stood by my denial.
Since you have already mentioned it to their parents, my next step would be to decide how else I want to handle it. Do I want to ask the kids again? Do I want to say no more sleepovers until this has been resolved? Would I just try to watch the kids better next time? etc.
I don't know that I would trust them alone again, or maybe I would just let it drop, but that's a decision for you, as it's your family and you are the one with the family dynamics and kids' friendships to think about.
I wish you luck, and hope you can get this figured out! At the very least whenever you have sleepovers again, you can remind everyone to leave the dogfood out of the people food for breakfast!!