I would go to the toilet with her each time, if you can get her to go and pour water over her vulva as she wees. It'll dilute the urine and hopefully help it not to sting. Also make sure she is drinking plenty of water so it doesn't burn so much.
I think it'll probably hurt regardless of whether she uses the toilet or a nappy. It's probably easier to manage on the toilet actually...

Poor poppet, hope it heals quickly.