Over the years, Dr. Lisa Kelch and I have noticed many cases of "pull-up dermatitis" in our office. Although we see it commonly, after a literature search a few years ago, we realize it is not something previously mentioned in the medical literature.
We note that quite a few 3-5 year olds who are wearing big kid underwear during the day but still wearing a pull-up at night will have a rash on their bottom. It is often rough small to medium sized reddish bumps. They sometimes can itch but do not always. They seem to completely disappear when the pull-up is no longer worn at night as the child gets a little older.
Although it seems as if changing to a different brand of pull-up would help, it rarely seems to help. We have found that doing a layer of Aquaphor(r) Healing Ointment at bedtime helps. It also may be needed again in the morning. If that does not help the itching, doing a thin layer of 1% over-the-counter hydrocortisone cream in the morning every few days can help.