I'm with Ben, and have a few "also"s:
--Target carries Champion-branded technical tees and underwear: coolmax and other synthetics that breath well, dry fast and don't stench up. Boys' sizes often around ten bucks a piece. Some of them are better than stuff that costs three times as much elsewhere.
--Target also carries Starter brand. Synthetics are so important for the tee and underwear.
--The Nylon Switchbacks from BSA, not the great-looking cotton duck version that looks so good at Court of Honor. If not the BSA pant, then REI's kid sized pant is a great product AND you can sometimes score them in Scout green.
--Fox BMX/MTB gloves are great for use with hiking poles and for lightweight, just-take-the-edge off insulation, and they dry fast. They're also a good under-layer for warmer gloves, but way more durable than "real liners". The guys can rock climb, etc with them, too. I spec Fox because they make sizes that fit kids, full-finger of course.
--Sierra Trading and REI will both have sales on the Marmot PreCip and/or REI's house-brand kid-sized, lightweight, packable rain shells. These are great and make a very functional outer layer for the fleece or just the tee when hiking in the rain. They pack down and get double use at camp throughout the year. We got ours for $36, and this is very justified by the savings on inner layers at Walmart and Target.
--kids are light: rolled foam pads or the z-pad are perfect because they're light, inexpensive and adequately cushy. There's a $12 version at Walmart that is good enough for summer and warmer spring/fall camping, for sure (the "fancy blue one" as opposed to the $7 cheapo blue one).