Evolution is the idea that living things change over time. Think of it like a caterpillar changing into a butterfly. Over a long time, different animals and plants change, sometimes just a little bit and sometimes a lot, until they become new and different species.
Development is how animals grow and change as they get older. When a baby is born, it can't do much besides crying and eating. But as it gets older, it learns new things like walking and talking. This is development.
Evolution and development are related because evolution happens over a long time, millions of years, and development happens over a shorter time, a few years for most animals. So if one species changes a little bit over time because of evolution, the babies of that species will be a little different, too, and they will grow and change a little differently because of the changes in their parents. Over time, those small changes can add up to big changes and new species.