Set an alarm/timer for 90 minutes, 60 minutes or however long it is between times that she goes to the bathroom. This takes a little time, but is an AUDIO reminder that she needs to STOP whatever she is doing and go...even if she says she doesn't have to. If she refuses, whatever she is doing STOPS and she does not get to continue playing or watching the movie or whatever, Give her some responsibility. When she hears the timer, alarm clock or whatever, she knows this is what it means. It will get her into the habit of going on a somewhat regular basis.
Paus the movie, leave all toys, etc exactly as they were, etc. until she goes and returns. You need to remind her that in spite of having fun, she has to take the responsbility to do what needs to be done. We started talking about responsibility and priorities at 4 so that they can start making good choices.
Talk about good choices and bad choices and the consequences and/or benefits of each. Don't know if when you let her have the toys again after you've taken them away, but if you give them back.....even a week or so later, she realizes the consequences are short lived and she gets toys back anyway, etc. Once it's gone.....it goes to Goodwill, a charity, or whatever. If she wants to play and keep what she has, then she has to take the responsibility to go to the bathroom in order to keep it. Otherwise, it's gone for good. Giving it back after a certain amount of time won't work. It needs to be given away or EARNED back.