PVC pipe, foam pipe insulation, and duct tape are the primary materials. Half-inch PVC is best for the younger ones, 3/4" as they get older. But be careful, because if you are making something longer than 36", that half-inch stuff can get whippy.
18-36" is best. As in all whapping games, your older or stronger children will have some advantage. They will have less advantage than in most other games, however. You can equalize things a bit by giving the younger child a longer, but lighter sword. Or you can make only one shield, and give it to the younger one.
Don't run the PVC all the way to the point of the sword. Leave about 4" that's just foam insulation wrapped with duct tape. Ditto the sword crosspiece, which should have a 1" foam overlap on each end. A pommel at the bottom is sufficient protection.
Wrap the duct tape in spiral fashion, overlapping it enough so that the final amount of tape is two layers. One layer tends to split. Three layers makes the sword stiff enough to really sting -- too much.
Kids can still get hurt with this, as with any game, but even bruising is unlikely. A stinging smack that brings tears to your eyes is usually about the worst of it. In winter, the cold stiffness of the "blade" is offset by the extra padding worn by the combatants.
I have seen people attempt scimitars, pikes, two-handed axes and the like. Why bother? These swords aren't going to look particularly authentic no matter what you do. They're for play, not reenactment.