My retainer in a child's mouth.

Okay so my younger sister (7yo) had a couple of friends over at our house today. I left my room for FIVE minutes and when I came back her friend (5yo) was in my room, holding my retainer. My sister later informed me that she put it in her mouth too. I'm never putting that in my mouth ever again, and it's expensive to replace (50€ or so). I'm SO mad right now. Like come on. I was gone for five god damn minutes. Am I supposed to lock the door when her friends are here? I don't know much about kids but I feel like at 5 you should know basic rules like not going into a stranger's room and mess with their stuff. Smh.

Edit: I get that I don't really need to replace it. I was really mad last night and felt grossed out. If anyone knows how I can clean it really well please let me know.