Dan,
The one recommendation I can offer is to not make the fowler too small. I just recently finished a very small rifle for my 3 year old grandson.
https://americanlongrifles.org/forum/index.php?topic=61759.0 Excerpt:
To show scale ....one of my full size rifles and this little one......
This also gives a sense of how slender the wrist is........
He will not shoot it for quite some time to come and he will grow out of it fairly quickly. Here is a picture of me shouldering the little rifle. To get my head down to the sight plane I need to get my "snot locker" almost jammed into the trigger guard....and I will probably loose some eyebrow hair when the pan ignites. The pull is only 10 5/8 inches.
On the other hand, a while back I finished a ~ 3/4 size rifle for another member on this board and, if I build another sub scale gun, I will build it this size instead. As shown above, the tiny rifle I could shoot as an adult but will need to sacrifice my eyebrows and some additional hair line. The 3/4 scale gun I worked on would be great for a young person but would still be quite easy for me to shoot as an adult.
https://americanlongrifles.org/forum/index.php?topic=37317.msg358295#msg358295Excerpt:
A comparison between the little rifle and what I consider to be a full size rifle. The larger one has a 42 inch barrel and the little one has a 30.5 inch barrel. The pull on the little rifle is 13 inches and on the large one, 13.25 inches. So the little rifle can easily be shot by an adult but is easy to handle and light enough for a young person or a petite lady.