I buy Walmart camoflage, they want to look like dad.
Plenty of layers, they get cold quick, but they get hot quick too.
I try to find the clothes on closeout at end of season and buy 1 size too large so they dont outgrow it by next season.
Make sure they have decent waterproof outer layers, they dont like getting cold or wet.
Never skimp on boots or gloves.
You can also look on sites like craigslist to find better quality used clothes, but so far the clothes have only lasted 1 season before they outgrow them so I'm not overly concerned about longevity.
Best thing is to keep them happy so they want to come back again-- I always make sure to pack a little bit of their favorite candy for when grumpy time rears its head.
An inexpensive but well fitting small back pack to carry their own food, hand warmers, water, small set of binos, and a book to read for when sitting to keep them quiet.