I don't have any relevant experience with taking kids that young out; I waited until my own nieces and nephews were about 10 - at that point, I carried the tent and cookset, and they carried their own sleeping gear, personal eating gear, etc.

However, I have read and heard from others that, even with very young kids, you want to let them carry a small pack with a bit of weight in in - perhaps a kid-size backpack or a decent daypack with a hipbelt, in which they carry a pint water bottle, a day's worth of snacks, a poncho, their sleeping bag/pad, and maybe a stuffed toy - say, 5 or 6 pounds of stuff. It makes them feel like part of the adventure to carry part of the load.

Or so I've been told.