Most reenactors make their own uniforms by hand. There are some sutlers that make things, but they are expensive.
The Brigade of the American Revolution, a large organization of reenactors has patterns for sale on their website. Unfortunately they are probably way too big for a young nephew.
Here's the web link for the Brigade- http://www.brigade.org/barmerc.html
The patterns are accessible at the very bottom of this page. On the Women's page there are some patterns for children. You'll need to ask questions on that site to find out more.
To Join the Company of Military Historians click here