Apparently there was a pattern in one of McCall's Quick Quilts magazine issues:
http://appalachianquilts.blogspot.co...-few-baby.html (scroll down to 2nd quilt)
If you scroll down on this web page, you can see how someone designed and made a lego quilt:
http://thecraftyquilter.blogspot.com...1_archive.html
Here's another idea:
http://stork1.blogspot.com/2012/08/lego-ish-quilt.html
And this one:
http://www.weewonderfuls.com/2011/03...ego-quilt.html
Also found this older QB thread:
http://www.quiltingboard.com/main-f1...lt-t53201.html
For a 10yo, I would be tempted to create "pockets" around the border of the quilt where lego pieces could be inserted. Velcro probably wouldn't be great in a quilt, so would have to figure out some other way of keeping the pieces secured in their pockets when not in use.