It is simple to make a knight's tabard for your knight's costume, and your child can help in the design.
You will need:
An old white sheet or white fabric
Scanner and printer
Printer transfer sheet (for your own coat of arms design)
Red ribbon (for St George's Cross / Crusader)
Measure across your shoulders, and then again from the top of your back to just above the knees. Fold your fabric in half and cut out a rectangle with the first measurement (the width) along the fold and the second measurement as the drop.
Cut a very shallow neck hole along the fold as shown in the daigram.
Decorate your tabard. We used red ribbon for Sam's Crusader tabard, below, simply gluing it to the front.
For Jack's tabard (top of page and below), we scanned in his coat of arms design and printed it onto a printer transfer sheet (used for Tshirts etc). We then ironed it to the front of his tabard.