Have you talked to your kid’s doctor about getting your kid’s pulse oxygen tested while the kid is wearing a mask?
My three and seven year old grandkids wear masks at school and preschool without problems at all. Masks increase dead space ventilation which has no effect on oxygen saturation in healthy people. The child may increase their respiratory rate a few breaths a minute to blow off the rebreathed carbon dioxide. If your child’s oxygen saturation drops when they put on a mask they have other problems. The mild hyperventilation from wearing a mask can actually make the oxygen saturation go up a percentage.