If you get into weight pulling don't do any serious pulling until one year old.you could hurt the dogs legs or hips. They can do it ; they love to work.I would just get the dog used to the harness; and teach command to pull and form. Find a book on weight pulling. Good luck bill
How much the dog pulls in weight pull depends on many things: their training history, their confidence level, the trainers experience, the type of surface they are pulling on, the equipment, how the weight is stacked, weather conditions, etc.
Weight pull is different than carting. Weight pull takes place over a short distance, typically 16 feet. There's no break on the cart or sled, a human breaks when the dog has completed a pull. You need to have someone there to break the cart or your dog will get run over.
Carting (aka drafting) has a special set up that stops the cart when the dog stops. Dogs also need to learn directional cues and they will pull over further distances. If you want to have your GSD pull kids in a cart, this is the activity to look into.
Here's a decent video that includes GSDs learning