I don't have experience with child seats, but do have experience with rear mounted racks, which I once loaded with more than 20kg on a hybrid bike (mtb frame with rigid fork and 26 x 1.5 tires), and the weight over the rear axle was enough to cause unwanted front wheel lifting off the ground while I was off the saddle, so, I agree that a front mounted rack would be a very good idea for balancing weight distribution.
I used this bike for parcel delivery and was generally very good for that, the low MTB gearing made it good for hill climbing, at least for moderate inclines. It's major weakness would be this 18 Kg load limit that did not originate on the rack but in the geometry, since the chainstays where rather short, which forced the position of the rack very close to the saddle, so larger items would overhang the rear axle quite a bit.
A bike with a longer wheelbase is likely to have longer chainstays and that allows for a rear rack that puts the weight in front of the rear axle. I later acquired a dutch Gazelle 28" bike which had much longer chainstays and a built-in rear rack. This bike could be loaded a lot more without the mentioned issue.
I also had limited experience with a cargo bike (Only for reference: search the brand "bakfiest") and can confirm that the low center of gravity makes it very easy to handle heavy loads. Also, the big "bucket" makes it very flexible regarding different sizes and types of loads. You can even carry adult people quite comfortably, and, of course, there are child seats designed to work with them. Some cargo bikes do have a "step trough" design which makes it comfortable for shorted riders and most also have a "dual" kickstand with holds the bike upright while stopped, so it is very easy to load an unload.