Not sure if you're referring to the Julbo's (the Loopoings retail at $32 but I think the Bubbles retail at $40), but Julbo's have very high quality lenses in them in. They won't have irregular surfaces and distortion like cheap lenses, which can cause eye strain and fatigue. They also offer anti-glare protection which cheap lenses will not. Cheap lenses can be made of triacetate or acrylic that don't absorb 100% of UV rays. Cheap lenses can absorb as little as 40% of the UV rays. This means that they will dilate the pupil, due to the darker colour of the lens, and then allow MORE UV rays into the eyes than you'd have if you weren't wearing the sunglasses to start out with.
Just because a lens has a sticker which says it blocks 100% UV rays, does not mean it is so, unless you're getting high quality lenses from a reputable company. If a person decides to stick with cheap sunglasses, you can take them to your eye doctor and get them to test how much UV radiation the lenses block, then you'll know if they're safe for your child. Of course there is still the distortion, lack of scratch-resistance and lack of anti-glare coating on cheap lenses.
Just saying, there is a real reason for the higher cost of some glasses. There are also some higher cost ones that are more just because of a designer's name though, and can have cheap lenses in them as well.
ETA: and quality sunglasses aren't usually made in China like the cheap glasses in big box stores. For example, Julbo glasses are made in France, Maui Jim are made in Italy (MJ doesn't make little kids stuff; just another example)