I had a little boy in my preschool class that was about 30 months old. He had been wearing diapers and never made any movement toward wanting to wear underwear. I had a routine of asking children when they were on the changing table if they wanted a diaper or "big boy pants" which I always kept a large supply of for the class. In this case they were Spiderman underwear. He looked at the underwear and said he wanted to wear them instead of a diaper. So I put them on him and off he went to play. Normally I would then check with the child throughout the day to have them use the bathroom but in this case I forgot. In fact I ended up sending him home with his mother and forgot to tell her what he was wearing.
Two hours later I got a phone call from his mom. She said that she thought I had her son confused with someone else. I asked what she meant and she replied that her child was not toilet trained but he had underwear on. Then it hit me. About three hours had gone by since I had put the underwear on him and I was sure she was dealing with a mess. I apologised to her about not letting her know and asked what had happened. She explained that after school they had gone to the mall. I was really getting worried as I could just imagine what had happened. Surprisingly he made it through all of that dry and they were now at home and she wanted him to take a nap, but he would not take off the underwear and put on a diaper. She explained that she was not ready to make the change as it is just easier for her to have him go in the diaper rather than to look for a bathroom for him.
Interestingly that was the last day he wore a diaper. Not that there were no "accidents" but within two weeks he was fully through the process. Sometimes parents are not ready for the changes that children are ready for.