Dd (2 years 8 months) wants to wear knickers not nappies. She can hold her wee in for hours, until she's pressing her knees together and really grouchy with discomfort. Then she gets upset when the inevitable happens and she ends up standing in a small pond. But she won't wee on the potty or the loo. She sits on both quite happily, but she won't oblige. Any suggestions?