I saw a blog about making chairs and tables for children from cardboard night before last and forgot to pin it :-/ Went looking and can't find it but found some links that will hopefully help.
http://voices.yahoo.com/cardboard-box-furniture-kids-###-...
http://www.examiner.com/article/eco-friendly-life-sized-c... - this site has a picture of the chair so you see what I mean, but they're advertising kits. The blog I read said to use a box 12" x 12" for the base/seat of the chair, reinforcing it inside with cardboard in an "x", adding boxes for the back and sides in appropriate sizes and gluing them together. It showed the chair and table covered with paper of some sort that could be changed at whim. If I make some for my guy I'll let him color them with crayons, markers, stickers, etc. A table can be made with 2 different sized boxes, one longer for the base and a larger/squarer one for the top. Hope this helps :)