Single most common cause is not drinking enough:
Concentrated urine irritates the bladder, making it spasm more easily
Concentrated urine irritates the urethra, making it harder to hold onto urine
Sitting in wet knickers irritates the vulva, and the urethral opening, making it harder to hold onto urine
Children who are worried about accidents drink less...making accidents more likely
If infection ruled out (which sounds like it is):
Step 1 - drink enough, and encourage to pass urine regularly
Step 2 - no fizzy drinks, no citrus drinks, no caffeinated drinks
Step 3 - check wiping properly, changing promptly, ban bubble bath, and no sitting in bathwater with shampoo in it
Step 4 - make certain not constipated - if constipated make sure to treat for long enough for the cut to recover
Step 5 - if above all done, and urine now nice and watery, but problems persist, then practice "count of 5" (holding on for a count of 5 when desperate to go but already in the toilet)
Step 6 - once a week, measure urine volume when desperate, to see bladder capacity increasing
A number of these steps can helpfully supported by a star chart, though one at a time: to encourage drinking enough, then to for going for a wee first time told, then for going spontaneously, then for dry knickers for portion of the day... And if the above doesn't do the job, then referral onwards might be useful.