If this happened at/just before home time, I can see how it could easily have been missed.
There were three adults in my Y1 class today at home time. I was focused on my 1:1, who finds the transition from school to hometime difficult, the other TA was comforting a girl who was sobbing because she'd lost her coat in the cloakroom and helping her look for it, and the teacher was by the sink dealing with a nosebleed.
We were all in the classroom, all keeping half an eye on the rest of the class (who were sitting quietly watching Numberblocks because they were early back from assembly) whilst dealing with our specific issues, but something small could have been missed if the child didn't speak up.
If they'd been back late from assembly (which will almost certainly happen tomorrow because its Friday) then it would have been even easier to miss something low level, because we would all be rushing to get them ready and out of the door on time, as parents (understandably) complain if they're late.
By all means ask in the morning, it may be that the other children saw what happened and can help the teacher get to the bottom of it, but unless this happened much earlier in the day and he was left wet for hours, there's really not grounds for complaint. If it were our class, we'd apologise, do some investigating, and feed back to you at pick up.