|Please know that I am just a lover of writing and reading and my opinions are just opinions with no professional background so please take with a grain of salt. This is an intriguing story and I believe you could make it better with a little work. I love that Carlie wants to fly, and that she gets her wish.
*The first thing needing explaining, even to a child, is Harry's wings. Is it a magical farm? Is Harry magical? Carlie envies the birds, why does she not envy her friend Harry the horse who also can fly around?
*Secondly, Carlie thinks everyone else is important except her, but later we find out that she knows that kids like Jasper need her milk so she is important. So either she knows but doesn't consider it as important as her friend's contributions in the beginning, or she needs to find out in a bigger way when she does find out. Like she could overhear Jasper's mother telling him how important it is for him to drink his milk or something.
*Third, how did Carlie get her wings? More magic? A fairy's wish?
*Fourth, Michael down the road is a mystery. Perhaps just say Georgette the goat is still helping the kids who need her special milk.
*Lastly, for kids you might simplify a bit. Less explanation of things that don't contribute to the story, and also maybe always call the friends by their animal name too Henry the horse, Georgette the goat, so the kids wouldn't need to try and remember who they were.
I apologize for this being so long but I thinK you have a neat idea and would like to see you smooth it out.