POOP EVERYWHERE!!!

My so and I have been short on money so my friend from work told us her parents had a room for rent at their house. We have been living there for 2 weeks now and everything has been great, their family is big with lots of kids but it doesn't bother me or my so since we both get off of work pretty late anyways so we are never around to really see any of them... tonight my so and I got home and walked into our bathroom (that is in the living room hallway) only to have myself step in poop.... I freaked out turned on the light to discover even more poop on the toilet seat and floor... I try and stay calm since I know that they have a lot of toddlers not potty trained running around and it was probably one of them. And since no one other than my so and I were awake it wasn't like we could ask anyone or ask them to clean it up... So I got some cleaning supplies and got to cleaning myself... My only problem is, how do I keep this from happening again or let them know that their kids decided to have a poop party in my bathroom? My so told me to just not say anything and if it happens again then we can ask them not to use our bathroom but I was thinking I should I write them a note on the bathroom door and just say "noticed a little mess on the toilet seat last night, please remember to clean up thank you!" or something like "noticed a little mess on the toilet seat last night, no biggie, there's toilet paper and wipes under the sink next time just in case" I don't know what to do, I don't even know how to go about talking to them about it... Advice please! We really like the family and I work with their daughter so I don't want to embarrass anyone or make living there awkward, but I still don't want to come home to a poop covered bathroom