When you color a picture, sometimes you need more crayons to make it look even prettier. These extra crayons are supplementary because they help make your picture complete and colorful! So, if you have a red crayon and you add a blue one, the blue crayon is supplementary because it adds more color to your art.
Think of a sandwich. You have bread, but you want it to taste better. You add some cheese, lettuce, and tomatoes. These extra ingredients are supplementary because they make your sandwich tastier!
Imagine building a LEGO castle. You have a bunch of blocks, but to make it taller and more exciting, you add more blocks. These extra blocks are supplementary because they help make your castle bigger and more fun to play with!
In a story, you have a main character, but sometimes there are other characters who help tell the story. These characters are supplementary because they make the story more interesting and complete. So, supplementary things are like helpers that make something better or more complete!