I just started this course too. I'm a little bit unclear about the secondary story as well. I had a hard time identifying secondary stories in many of his examples. A possible secondary story for this piece would maybe be the dog being happy with his tongue out and looking at something blowing by him with interest? Or maybe the birds are flying closer to the dog and are surprised there's a dog in their territory? Or maybe a kid with goggles and scarf on a playground airplane in the background? Those suggestions would completely change the tone and the focus of the piece though. Maybe one that would keep with your current mood would be to have the dog trying to keep his toy from blowing away, so you have one more element in your chain reaction. It's so tricky to find the balance between what will enhance a piece and what will distract from it. Currently I do think your piece is clear, there's fun interest in the chain reaction, and nothing seems to distract from it, so you are successful on many fronts.
My only suggestion at this point would be to reconsider the gesture or expression of the girl hanging on to the fence. She looks a little nonchalant and relaxed to me. Maybe more tension in her grip- maybe it looks like her hand are about to slip- maybe her body is at a sharper angle or her expression looks more strained. There's probably a few ways to go about it, but I think it would help the viewer relate to the tension of the moment.