I can see why you are puzzling over it because it's hard to decide! With the first one, I think that's the one that stood out to me the most when I first glanced over the post. I think the idea of the grasshopper occurring throughout the book is cute -- I noticed a lot of picture books have an animal character that interacts with the story and adds extra visual interest. I'm thinking in particular of the little bird in Alma and How She Got Her Name. But I also think your concerns with the spread are valid.
I agree with you that the second spread appears too flat. I think it's because the line of the swingset cuts diagonally across the page. This might work if it were more zoomed in so that the figure was larger and that diagonal line wasn't so prominent? Because I see how drawing the child in this position is simpler and can pop really well.
For the third, I think if it feels boring to you on a gut level you should try something else. I fall into that trap a lot where I design something and I think "well this isn't exciting but I can imagine it looking good" and then it looks awful because I don't have that magic spark when I am drawing it!
I like the 4th one too! The tilted background emphasizes the fast movement of the swinging, and I see the splashes and raindrops making really interesting shapes on the page. The only thing I would change about it is to move the child further from the middle line so they don't get caught there.
Really cute baby sketch, I looove those pudgy little legs and tiny shoes. Good luck with this!