I think how much they can carry depends more on height than weight. Not sure.
But my own feeling is to carry everything until they are about 2/3 your height. Good rule of thumb. I just made it up. <img src="/forums/images/graemlins/wink.gif" alt="" />

If you want to go on a really long hike it is best to carry their stuff. If you want to go on a shorter hike, more towards camping, then it is not so critical. My own feeling is that, because of the way we evolved as a species, young children are ideally suited for long distance walking, like 20km per day even in rugged terrain. They're not the best pack mules, at least not until they gain some height. But once they are too big to carry, by design, they should be able to keep up. If you think about it it makes sense.

It is hard sometimes finding clothing systems that don't overburden them, so start with clothing. Find light weight clothing solutions, same as for an adults with no 'extra clothes', just many layers within one clothing system appropriate for the worst weather. Finding lightweight shells is tricky sometimes. You can save alot of weight and bulk in shells. You can sometimes take a small adult wind shell with a hood and shorten the sleeves for a very packable 4oz knee-length kids anorak. Footwear can be tricky also. Kids don't need heavy boots, but it can be hard finding something for snow they can still walk in. Often better to tow them in snow, and have them do their running around when you stop. Sleeping solutions are very tricky also. There are some good childrens sleeping bags but they don't come in enough sizes. I think it is generally better anyway to have everyone sleep together in one lightweight tent with blue foam pads to cover all the sleeping area, and then to use a combination of light sleeping bags, lightly woven wool blankets, and light overquilts, in order to find a weight efficient solution minimizing total surface area, usually with adults on the outsides and kids in the middle. It might take some work to build a solution customized to your family and with enough warmth for the season you are hiking in. When hiking with kids solutions can get complicated, and keep changing. For kids, the stuff in stores is not generally geared towards less encumbered longer distance hikes, but it is easy to make or modify or make do. Frankly I've forgotten what worked best for Margaret at that age, but if you keep having fun you will keep working something out, from one year to the next.

I will post something once I set her up again for this winter. Might even get her on cross-country skis this year. She has now turned 9 years, but still only 48" tall, if that. Still under 50 pounds I think. Its been awhile since I last checked. lol