The assumption in evolutionary theory is that organelles in plants,chloroplasts, where once free living bacteria that where internalized by a host cell. To explain this idea to kids, or maybe adults, think of this process as a house guest. You invite someone over to stay for a while. They are a very good cook. They make brownies, and cakes and pies and cookies. And they do it with using their own supply of sugar that comes from no where. The host, being someone who likes to eat this stuff,most of the time, will not want their guest to leave and will adapt the house to accommodate their guest.