Usually they are thicker and have a much thicker crotch area so they can hold a little bit of pee if the child wets.
I bought some and they were a huge waste of money for us. The extra thickness in the crotch was poly fiber fill - that's right - plastic fibers. They didn't hold anything. The pee just ran right through them, down her legs and into her shoes. You can put plastic pants on over them to keep that from happening but those trap all that heat and moisture next to the skin - my daughter had a diaper rash after just half hour of wear with NO pee.
In then end, I went with the naked method and then with panties. I do use pullups but only for long trips out when I'm uncertain of where a bathroom will be or if I'm leaving her with someone and I'm not certain she will ask for help to use the bathroom.
at 10:43 PM on Mar. 2, 2009