Question
What are small round greenish and about 1/4 inch tall that grow on the leaves of my trees. Walnut and oak trees.

Answer
Donna:
I suspect that you are referring to an insect gall- perhaps an insect called phylloxera.  This is one of many gall insects that can affect trees. Use "Google Images" to search for a photo to compare.  Usually they don't impact significantly the overall health of the tree. Be sure to rake up and destroy fallen leaves in the winter since these critters may survive over the winter on these leaves to come back next year.

Regards
Steve