My 5 yr old girl is still having occasional wet underwear events. We have talked to her, we have rewarded her for days when it doesnt happen, we have remindered her - eg always go to the bathroom at break time - to get her into the routine, we have talked to the school about reminding her. But even with this 1 day every couple of weeks she has an accident.
Don't get too stressed about it, its not that they don't have bladder control its just that they get so caught up in what they are doing and leave it until the very very very last minute!!! My son (5 on Sunday) does this too. Just remind them when they have an accident to not leave it sooo late.
It happens. Don't put too much pressure as this will have the opposite effect. Fundamentally, kids this age want to be "good" so they do motivate themselves if you give gentle hints.
She may be sensitive to dairy or too much sugar in her diet.
Also, when she is peeing, ask her to stop the flow part way through. This will help train her to hold her pee longer.