Aphids damage the leaves of plants and suck the sugary photosynthesis juice. The aphids excrete part of the sugar as honeydew, which is often eaten by bees and ants. In return, the ants protect the aphids and bring them to new leaves.
Explanation for children: Aphids are small white dots that are sometimes visible on leaves. They break the leaves because they contain sweet juice that they want to eat. But they can only digest part of the juice, the rest they excrete as sweet "honeydew". Animals such as bees and ants like honeydew. They eat it, and in return they protect the aphids.