New article - After 2 hrs, ZnO sunscreen loses effectiveness; becomes toxic, page-10

The chemical UV filters react to form toxic by-products. The same thing has been seen with Chemical UV filters reacting with the Chlorine in swimming pools to form toxic by-products. Zinc Oxide itself is inert and naturally occuring, with diaper rash creams for babies using a concentration of up to 50% Zinc Oxide, while most sunscreens use only 25%. Probably best you start off by reading the paper in full and not just the headline.

Meanwhile the FDA reccomends kids do not touch the chemical stuff at all for fear of side effects.
https://www.10tv.com/article/money/consumer/fda-suncreen-warning-for-young-kids/530-86d159e7-b091-4fb6-be24-98e515555682

“There are mineral sunscreens that are much safer, like titanium dioxide and zinc oxide are the go to standards when it comes to sunscreens," she said. "They are very safe, they don’t cause rashes and you can use them at virtually any age.”

Titanium dioxide does not achieve broad spectrum UV protection so you are forced to include Zinc Oxide in the formulation to comply with broad spectrum regulatory requirements. At that stage would you just go with a zinc oxide only formulation?