C.M.
I think you are on the right track. Sounds very simliar to my daughter (who's been in underwear for 9 days now). She's had 2 accidents but the other nights when she wakes up, I take her to the bathroom so I think that has helped. Some kids bodies just do not signal to them to wake up - I do not think my daughter would get up on her own and go to the bathroom. The two times she peed, she slept right through it. So give it another few days and if she is still peeing, tell her that her body is not ready for big girl underwear at night and she can wear them over her diaper/pullup.
Good luck!