Mel,
I guess perseverence, maybe try to explain to her she can make a big persons decxision, Would she prefer to wear nappies again, or does she really want to be a big girl & wear undies? Maybe you can get her to tell some people (aunties, grandparents, friends etc, etc) about how she wears undies now, really amke it a special thing...
Also I know Maddy was tiny for her size 2-3 undies too & they were SO baggy... I went to Springvale & bought underwear from the Asian shops there, as they were smaller.
I also got character ones from the Asian market stalls at the markets!!

Good Luck, I am not sure I have offered anything helpful tho, I'm sure your during everything already!

Good Luck!!!