Any of those breeds could be trained to pull a sled. Generally a dog can safely pull three times it's own body weight. However, that's over a fairly short distance and not any great speed.

Sibes are generally good with kids, but they are highly active and can be a bit much for toddlers. Like wolf-q said sibes are hard to contain and need special fencing. Most huskies don't come back if they are off leash.

Malamutes can pull more weight than a husky. However, they are also much larger, need more room ect. They have the same fencing requirements and GOOD LUCK getting a mal to run and pull at the same time. My malamutes at best trot when they have a load on. My purebred mal refuses to pull at all with any kind of weight on. He's lazy!

You also need to consider any other animals you have. Both malamutes and huskies tend to be bad with smaller furred critters. Malamutes also have a tendancy toward dog aggression, especially to same sex dogs.

I've never owned a Bernese Mountain dog, but friends of mine have them. They are trained to weight pull, not fast, but they can move a lot of weight. They are much easier to train than huskies and malamutes. They seem to lack that stubborn streak that the other breeds have in spades! The ones I have met have been good with kids and generally laid back dogs. I'd love to own one someday.