All sunglasses are not the same. Effective sunglasses should block both UVA and UVB radiation. Large lenses that fit close to the eyes are best. Those that block visible blue light are even safer. Thankfully, all sunglasses block UVB radiation. A great many sunglasses, however, do not afford UVA protection. Expensive brand names and polarizing lenses are no guarantee. The sunglasses must be measured to block 99% to 100% of UVA or UV400 (400 nm is the wavelength of UVA radiation).
Ordinary sunglasses make the situation WORSE! The dark lenses cause the pupils to dilate, allowing more of the dangerous UVA radiation to damage the lens and the retina.
Of course, children will break and lose sunglasses with remarkable rapidity. Thankfully, suitable sunglasses are quite cheap. Expensive brands may be more stylish, but they confer no advantage when it comes to eye health.