OP, I'll tell your our experience of night time wetness.
My 7yo DD has been dry since the age of 2.5yo. However she recently had a 6 month lapse where she was wetting the bed almost nightly.
We went to the GP and the school nurse, but no underlying reason could be found. As our DD was 6 at the time, we wanted her to be involved in deciding what we were going to do.
Initially we used the disposable bed mats (which are expensive, but did help protect the mattress and were very convenient) and DH/I would wash the sheets on a morning and replace them on an evening. It wasn't easy; I am a full-time student at uni 60 miles away and DH works 60hrs a week, but we were patient and never let DD feel as though her accidents were an inconvenience. It was not her fault, or something she was aware she could control.
We were told by the GP that they could prescribe desmopressin, but we were reluctant to go down that route. After a few months of wetting, DD became fed up and wanted an 'undisturbed' night, so she asked for night time pants. After a week of wearing them, she became night dry again. She has been dry for a couple of months now and hopefully we have come out the other side. However, we still have no obvious reason for the lapse.
What can I advise? I would suggest that starting school has affected her and it is likely that the accidents are related to that. I would go to the GP to rule out anything like a urine infection, however they will be reluctant to do any further investigations because your DD is so young. You could try night time pants for her, which are more similar to knickers than nappies and will ease the non-stop laundry. Most importantly though, be patient, consistent and calm; your DD isn't doing this on purpose and may well be distressed herself.