For anyone else reading this post, this is why we say CLOSE ALL TOILET LIDS!
It's not very pretty but I have a yellow "Post-It-Note" stuck to the inside of my bathroom doors reading "PLEASE CLOSE TOILET LID!"
You can also get some hardware mesh...the same stuff many wheel tracks are made of and make a toilet ladder. You just bend it so there is a bit of a lip, the toilet seat holds it down...that way IF it were to ever happen, they can get out.