how often big change water in swimming pools for children?

We have a large children's wading pool this basis is used daily on a fast. It takes a long time to fill and empty, and we have cover for the top. How long should I water and what the alternatives are for the disinfection sure (Our children are very young and they often swallow the water, so that no chemicals please), I thank you for your suggestions!

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Salt Water Swimming Pool Alternative: Ionization