My 9 year old still has very rare accidents, but up until a year ago, she was still wetting once or twice a week. I understand the frustration and having to get up in the middle of the night to change sheets, blankets, etc. I have a son who's 5 and stopped wetting at night at 3 years of age. We tried a bed wetting alarm with her and that didn't help either. The only thing that made a difference was time and patience. The fact is all kids are different. What's most often the case is that the size of the bladder hasn't quite caught up with the volume yet or the child is a very deep sleeper (I think my daughter is a case of both.) However, even if it is bladder size, the child needs to learn to recognize signals that she needs to go and to get up and go. The night time "pull ups" that they're selling now are a bad idea as far as I'm concerned EXCEPT on occassions like being on vacation, staying the night at a friend or relative's house, in other words, any time when she is not sleeping at home, then I think they're ok. At home though, put her in underwear, get a plastic mattress cover, and just be prepared to have to change a lot of sheets for awhile. The reason she's just going in her pull up on purpose and not going to the bathroom is because the pull up wicks the wetness away from her and she is not uncomfortable, therefore why expend the time and energy to use the bathroom? It becomes an excuse, a crutch. At 7 she's definitely old enough to know this is wrong! Wetting the bed in her sleep is not a cause for punishment, but doing it when she's awake most definitely IS. Take the pull ups away, store them for away from home trips, and put her in underwear with a plastic mattress. Make her wake up wet and uncomfortable for awhile and have to get up to come ask you for help. Maybe at 7 she's still a bit too young for this but you could try it...teach her how to change her own bedding and pajamas. If she has to get up and change bedding because she wet the bed, she may start paying more attention to her body and waking up in order to avoid having to go through all of that. Bottom line is though, avoid pull ups and let her grow out of it in her own time. It might take a couple more years, likely not, but it may. But pull ups are not the answer. Also, be sure to take her to the doctor and tell him your concerns and have him test her for any infections as sometimes even just a slight but long term lingering kidney or bladder or urinary tract infection that doesn't cause any outright symptoms can cause bed wetting problems. I would avoid any medications specifically for bed wetting that the doctor may want to prescribe though. These can have some pretty negative side effects.
Also, restrict drinks for a couple hours before bedtime. If she's thirsty, let her have a couple drinks of water, and also make her try to use the bathroom before getting into bed, even if she says she doesn't have to go. Another thing you could do is if you are still up a couple hours after she's gone to bed, get her up and take her to the bathroom to try again even if she went before bedtime. I had a doctor tell me once that this wouldn't help and would just "disrupt her sleep cycle" but we did it anyway with our daughter and it did help and it didn't seem to make her more tired or anything the next day. Personally, waking up wet and having to change clothes and wait for sheets/blankets to be changed seemed to me to be a disruption to her sleep and a worse one than a couple minute trip to the bathroom. She stayed dry most nights that we got her up to go and it may help "program" her to wake up around that same time every night to go, kind of like how you often wake up before your alarm clock goes off because your body's so used to waking up at that time.