If you can keep the kids out for a few days... Place a 5 gallon bucket on a step and fill with water to the same level as the pool ( I place a big rock in the bucket to keep it from floating) .. let sit for about 3 days. If the levels are the same you can rule out a leak. both will evaporate at the same rate.

being back east this may not be true but out here in Az with 110* temps and 7% humidity, I have to add water every couple days .