CW, First thing is be proud of that saddle!
Building a saddle for your kids-- I think is really special. Overall you did really nice work.
Just my opinion, but when you put your sons saddle on his horse you will have a different take on the skirt, my guess is that it will sit real nice.
Fenders are tough as far as being in proportion to the rest of the saddle when your little guy is 3 feet tall, it is what it is.
Again just my opinion-- seat profile looks good for a little guy. Great stuff regards Billy