My policy is 3 weeks accident free at daycare to start using pull ups. If they have accidents after they started using underwear they go back to pull ups and again they need to stay 3 weeks accidents free... My son was dry when sleeping and he was using underwear when we went for vacation. Since he had to sleep in the same bed than us I was not taking the risk of getting wet if he pee in the bed. So I put under pull ups on. He did not pee in the bed. When we were back home he used underwear again for sleeping. No accidents either, because he was fully potty trained. If this girl is not fully potty trained she will have accidents not matter what (using under ware or pull ups). The only difference is the mess you may have to clean.