I found a great way to get my 5 year-old involved.  She loves to paint.  I hate to assemble kits of brick buildings without the mortar being painted.  Instead of using acrylic, I pulled out the good ol' Crayola tempera paint and let her coat the building sides.  After it dried, she took a wooden clothespin and ran it over the surface, leaving perfect mortar lines in the brick siding.  Awesome!