We've tried those with some success. They make some for girls. I have her in panty covers they're made to wear under skirts(we called them spankies in high school) they are just like those, but they are really not a good fabric to use as panties. She's prone to infections.

This might sound a bit odd but I wonder if she'd be more comfortable in boys boxer briefs than girls panties. They're the kind that are fitted (not loose) but they have a slightly longer leg so they don't have elastic at the leg hole.