Green science are doing solar oven kitts for £10 ok there not the most amazing things in the world. Designed for kids to try and cook eggs with, not a bad idea if you want something practical for your little ones
I had a geography teacher in school that had a solar oven. He said it was a pile of poo, even out in Uganda where he was using it to cook a steak. Said it did nothing but made it warm and smell like off meat.