Usually have a big rear rack for that can carry both a single child seat for a small child and panniers at the same time. Weight ratings are often limited to around 35kg for the rear rack and 10kg for the front rack.
They will carry one or two children and or one adult sized person. due to their nature of being a bit shorter in length in the rear, it can be a bit close to fit a couple kids on the back so it is better suited for two under 5's or a single over 5. Weight ratings on the rear rack are usually around 60kg.
Due to the extended rear rack, this style of bike can accommodate one or two or three children. The Yuba can do this more comfortably due to being slightly longer. Weight ratings are usually around 200kg over the whole bike, rider included. The Yuba is the only long-tail we have that can accommodate up to Three kids or two kids over 7.
You can fit up to three kids in the front and even sometimes (in the case of the Supermarche) you can fit a kid on the back as well. Obviously this does depend on the size of your kids, and could involve a bit of tetris for several bigger kids like ours (6,9 and 11 at time of writing) Weight ratings are 250kg for the Riese & Muller's and 100kg in the front of the Supermarché and Bullitt.