1. Depends on what sort of linen we are talking about. I use 1 layer of high quality fabric with much success. If you are talking raw unwoven I have no experiencee with that.
2. Anything from glue to parafin wax ,paint, shelac, your choice . Cheap and easy is the way to go
3.Make it bigger and heavier than you think he/she needs . They grow into and out of them quick ,and it doesn't hurt the bow to be drawn a little less than planned.15# is a good starting weight for kids,say around 8-10 years old. Make the arrows good and light so it shoots fast and try to get it as close to center shot as possible because you'll have a bugger trying to spine for a 15 # bow.
Just my 2cents , hopefully others will add theirs so you can choose the answers that best suit you.
Cheers, Dave.