The most common cause of vulvovaginitis in children at this age is non-specific, irritation or inflammation.
And as she has excessive exposure to water so it can result in increase in moisture resulting in growth of causative agent like yeast,bacteria.
So she needs to keep the area dry and clean and avoid bathing,swimming for a while.
Keep the bath time less than 15 minutes.
Don't put any other soaps or shampoo into the bath water.
Wear cotton underpants.
Teach your daughter to wipe herself correctly from front to back, especially after a bowel movement.
Encourage her to drink enough fluids.
If her symptoms persist more than a week even with such measures then doctor may advise Oral medications.
Usually at this age,oral medications are not given initially as these medications are not so well tolerated by children.
I hope this helps.
If you have any further query then feel free to ask.
Positive feedback and bonus are highly appreciated.