I went back to the park and collected the leaf with these hoppers
and their eggs on it. When I was home looking at them, I saw this teeny speck moving on the leaf. Thinking it might be a nymph, I took its picture. To my surprise just now when I looked, it was a little tiny wasp! And now that I look at that linked photo again, I think that little yellow creature on the eggs is the same wasp, or one of its brethren. Is it parasitizing the eggs?