Happy endings are more fun when you've overcome obstacles along the way. Despite the kindness of Grandpa's "Don't worry", it's not a bad idea to include a few worries out, too. The reader will worry at implied dangers, whether or not you describe them. But it's better if you address some of them, so that the reader knows that the narrator sympathizes with his or her worries. Also, worries and struggles help us to identify with the characters and see some of their strengths. I think that even child readers (depending on the age) can handle--and enjoy the characters' encountering and surmounting some adversity.
I see some opportunities in StoryAboutKites. This may be pushing the story toward an older audience than you intend, but perhaps there are one or two things that would be useful.
Once Rose gets blown into the tree, John could certainly do some worrying. What if they're late to school? For the second time that week, and they've already been warned not to do it again? What if one of the branches scratches her and she has to go to the hospital and get patched? What if someone blames him and says he should have done a better job watching out for Rose?
Then there's Rose herself. Where her character comes across most clearly is in the brave face she puts on. Does she put on that brave face right away, or only when her grandfather comes? When the wind blows hard and the willow branches whip around, does her brave face waver? Even if she's not all that frightened, what does she do to handle her frustration?
Rose's grandfather seems awfully easy to find. What if John had to look for him a little harder--for example by flying higher than he's used to?
I'm half-tempted to suggest that the grandfather have trouble hearing John's call for help, but that's kind of stereotypical. But there's a very intriguing bit about grandfather's long years of experience with trees. To get jargony about it, maybe you could unpack that a little. What skills and strategies does the grandfather have to use? Does he hit any snags? Have to rethink anything? Just as Rose gets a lift--literally and metaphorically--from this experience, maybe Grandpa gets a lift, too. Good to know he's still got those tree-wrangling skills.
On a different note, I think with kites it might be nice to have some description of what they look like (even if their are pictures). How does it feel to be a kite? To float on the wind, to dangle a lovely orange tail?
And what's the relationship between Rose and John--what is it that makes them friends? People love stories about strong friendships and would probably appreciate a little more detail.
