The vest has to be a solid. Brown and black are the appropriate colors, I believe. I've only used medium-dark brown cotton, but it's supposed to be polished cotton.
The most commonly found material for men's vest backs is a brown polished linen or "holland". Polished cotton is used for many repros because holland isn't available.
I've seen so few children's vests that I would hesitate to suggest what was "common", but it occurs to me that children's clothing might have accomodated a lot more variations than men's. That said - if you want to make a colorful vest why not put the cotton plaid in front and the brown polished cotton in the back?
For the 14 yr old, I'd get him man's braces. You'll have to search the forum to get the right specs on those. The ten year old should be wearing them in the same style, but I confess to using poor boy suspenders for my eldest son, 9 yrs, for a lack of anything better or justifiably affordable.
A good pair of men's braces might cost you $70 or so, and some grown men confess to wearing "poor boys" as well. It won't be a problem if he keeps his vest on; and if he really enjoys the hobby, braces make nice Christmas presents