Like I said, a toddler's hygiene will drop throughout the day, just like every other sim. It doesn't have all that much to do with making it to the potty chair or not, as if they're hygiene gets low enough, it will say "Change Dirty Diaper" even if they don't have an accident. It's just how things work.
In real life, when you change a child's diaper there is much more involved than removing the old diaper and putting a new one on...in fact, if you only do that, the child won't be very comfortable or clean. Thus, it's easy enough to assume that when you change a baby/toddler's diaper in the Sims 3, there is more involved than just slapping a new diaper on them. You just don't see it all happening.