I had this problem with my DD.
I was very concerned and spoke to her doctor about it on more than one occasion. She assured me that this was more common than I would believe, people just didn't talk about it. She said that her bladder just wasn't keeping up with the growth of her body, and that it would catch up eventually, and this would be over. She said most kids outgrow this problem by age 8, but some it takes well into their early teens (puberty). She said that the worse thing I could do, was stress over it, because DD would sense my stress, hence causing her more stress. So, we just kinda dropped the issue.
I did follow some advise her doctor had to offer, and took away anything to drink after 6pm. If she gets thirsty after that, she is allowed to suck on ice chips only.
Right after DD's 10th B-day, she started staying dry more and more (always wore overnights). So when she got to the point that she was managing to stay dry for about 5 nights a week. We took the overnights away, I was afraid that this may be part of the problem. I was potty training my DS (3), and noticed that when he napped with undies on, and had an accident, it woke him immediately, when he had on a pull-up he slept on through. I wondered if this would work with DD. I laid out fresh pjs, sheets, and blankets, beside her bed, in case she had an accident. She only had to use them about 3 times in a 3 week period. Actually getting wet, when she urinated woke her up, where it did not when she wore the overnights. She pretty much retrained her body, and in about a months time, was almost accident free.
She still has an accident every now and then (about once every 2-3 months), but it's usually when I haven't realized the time, and allowed her to drink something too close to bed time.
Your child will outgrow it, it's only a matter of time. I hope I have helped.