Medieval wear for kids can be a whole different adventure! I hope to have some material to offer here soon.
For a quick fix, you can make a t tunic for a child by taking a solid, plan T-shirt - like from the craft department - and put some trim on it.
Another tip is that you can actually trim it with 3-dimentional paint!
Parents, you may find this Facebook group to be useful -
https://www.facebook.com/groups/339987266133717/
[Non-SCA Site Links]:
Here is a great class that has been taught in the SCA with some excellent photos and information for ideas.
http://www.cardinal-creations.com/Philippa/DressingChildrenInTheSCA.htm
And some other great info to keep in mind
A collection of related emails -- http://www.florilegium.org/files/CHILDREN/idxchildren.html