7 year olds are funny

I have been trying to enforce an English only rule in my classes at the hogwan where I teach. (A hogwan is an after school institute where you go to get extra teaching in some subject. Obviously, I teach at an English hogwan.) I have been trying this after I met some of the students of another friend who were only 10 years old and could communicate in nearly perfect English sentences. Anyway...part of this new campaign of mine is to make the kids say "bathroom please" for the younger classes and "can i go to the bathroom" for the older classes. (This is instead of just holding themselves and pointing, or saying the Korean word for pee.) So, I have the cutest group of 7 year olds. (They are about 6 in American age.) There are 9 of them and they are my favorite class. One of them, who used to be one of the biggest trouble makers, is now a dedicated student and is always trying to please. He got the phrase "Can I go to the bathroom?" mixed up with some other phrases, I guess, because the past few days he has been saying "Teacher! Can I do it in the bathroom?" I almost start laughing when he says it, but then I try to correct him instead. He gets it right when he repeats it after me, but then the next day he reverts back. Yesterday, I was telling my other teachers about his special sentence and I started laughing so hard I cried. Oh, I love those kids!