Any child over 2 years old (so adults too!) can start to wear goggles. For comfort, we recommend a dedicated age-specific-sized pair of goggles with a fixed nose bridge for children (i.e. not adjustable) with a permanent gasket (cushioning) around the eye socket (i.e. not the plastic ‘bands’ that can fall off).
We also practice going under the water without goggles, mainly for safety but also if you’d like an underwater photograph they’ll look much better without goggles