Absolutely not acceptable! The lav is for business like this, for anyone of any age. Even if there isn't a diaper changing table, bring a pad and do it on the top of the toilet lid.
It's horrible to think of someone exposing their fellow passengers to the sight, the smell, and the potential biohazard contamination. Also, it's certain that someone will sit in that seat on an upcoming flight and that creates an incredibly unsanitary condition.