I have not got children myself, but I have experienced this as a nanny.

I found that, unfortunately, most children do need to be reminded. They just get soooo involved in play that they forget.
So what I found quite helpful, was that i would tell the kids that they would get a stamp or sticker each time they went to the toilet. So maybe to start with you could remind your child to go to the toilet and tell her that if she goes without making a fuss, she will get a stamp or sticker afterwards. Then if that works, wait a week, and tell her that she is a big girl now and that you are going to make it harder and that she will only get a stamp/sticker if she remembers to go herself (of course you will have to keep an I on her and remind her if she hasn't gone in a while). This normally works with every child. You just have to make it fun for them, if that makes sense.

Good Luck with it all. She will eventually get the hang of it. Some children just take longer then others.