Well that's a tough question... First, how much can your rack hold? Because many racks are either rated for 55lbs/25kg or 77lbs/35kg.
What we say is, the Crate can easily hold 44lbs/20kg, the bags are comfy with about 15lbs/6kg in each then your whole system weighs about 75lbs/34kg which makes it nice and safe on a heavy duty rack.
The Child Seat is limited by the safety certification to 48lbs/22kg. Safety first.
The Klik itself can be sat on if you bike is sturdy enough. That's how we taxi people around ;)