In general, look for a suspension system akin to an expensive framed backpack (rigid external stays, good shoulder straps, thick hipbelt). Remember you'll be carrying a 20+ lb kid, plus water, snacks and diaper changing stuff. Fully loaded with a 22lb 1 y/o kid, 1L of water, diaper kit, snacks and small blanket, my rig weighs right at 35 lbs. Good load transfer is imperative.
A solid harness for the kid is important too. I'd look for a shoulder harness with sternum strap, and a waist belt holding the little one in in case I have to scramble (I've climbed Class 2+/Class 3 with mine) It's also really nice if they fall asleep, the harness holds them upright. Good ones will have a pad in front for them to sleep on. Also a hydration sleeve is super nice.
I have an REI Piggyback, its fantastic. Really really nice. If you can find one on CraigsList, snag it, as they aren't made/sold at REI anymore. I've been carrying my 1 y/o in it for 4 months and its all day comfortable for me, as well as for him. We've hiked for 3 hours, no stopping before.
Kelty makes a good alternative, but get the FC, not the TC. The TC doesn't have a good enough frame/suspension to be comfortable for you over long periods.
Some have a sun canopy, I think this is important, but opinions vary. The Kelty FC has one, as does the REI.
I'll elaborate more if needed, and post a pic of mine when I'm not posting from an iPad.