I just had a conversation about this with a dcd who thought his daughter was ready for underwear yet fails to tell me when she needs to go and does not poop on the toilet.
The child must be in pull ups all day. Period.
If this dcb eventually does poop on the toilet and tells you when he has to go to the washroom THEN I would allow underwear after a certain period of time However, if he still naps, he will be in pull ups.
Should he poop in his underwear, whatever I can get to fall into the toilet will be flushed and the rest goes home.