I really enjoyed reading your story, your imagination is so wonderful! The one thing that I would be mindful of is dialog. When your characters speak, remember to separate the talking from the lines that are being written, for instance,
"Miles eyed the log that Margaret had placed seeds on for the yellow bird.
“Look Margaret,” Miles pointed at the pile of discarded shells, and tried to sport a little humor.
“That bird eats just as messily as you do.”
They both attempted to laugh, but they each knew that they would feel uneasy until they were back in their own courtyard.
You need to separate the lines and also, in this same text remember that you do not have to repeat the same thing to get across your meaning, as in..."Miles eyed the log that Margaret had placed seeds on for the yellow bird." and then you write..."Miles pointed at the pile of discarded shells, and tried to sport a little humor." I would suggest saying, Miles eyed the discarded shells and pointing them out to his sister said...This way you say it all in one line instead of repeating. However, this is a great story! I really enjoy stories about how children can overcome obstacles on their own.