I bet there are children’s books about evolution. Maybe just explain that natural selection is when a mommy and daddy choose each other to make a baby. They might like blonde hair or blue eyes and so those features get passed down to the babies. Same with being taller or strong or smart. Maybe during certain bad times the physically taller and stronger do better so then the species has many more strong animals in it, because they survive. Or, maybe other bad times smarts are more important and they survive. Sometimes in nature there is a random mutation, like an animal might be better able to see at night, and that helps him survive. Or, they have thumbs, and that helps them survive. Or, their digestive track is able to get nutrition better. The ones in the group with the useful mutations make babies and the species grows in that direction with those traits. On some islands, like the Galapagos, there are animals there that don’t exist anywhere else because over centuries they were very isolated from other parts of the world.