Take sunscreen with you to reapply during the day, especially after your child swims or exercises. This applies to waterproof and water-resistant products as well.

Follow the directions on the package for using a sunscreen product on babies less than 6 months old. All products do not have the same ingredients; if your or your child’s skin reacts badly to one product, try another one or call a doctor. Your baby’s best defense against sunburn is avoiding the sun or staying in the shade.

Keep in mind, sunscreen is not meant to allow kids to spend more time in the sun than they would otherwise. Try combining sunscreen with other options to prevent UV damage.
Source:  www.cdc.gov