This idea is the reason we bought our first cargo bike, leading us to go into business as Cargo Cycles. Small children (or in our case, a disabled child) may not be strong enough to sit on the cargo bike’s standard bench seat. A car seat can provide much more support, and has a secure harness.
Attachment methods will vary depending on the design of your car seat. The type of car seat you use will depend on the size of your child, and what you have available.
My first car seat installation was rear-facing, using the seat from our car. The seat was installed “upright” against the inclined face of the front of the box which gave it a suitable recline to give Sarah the support she needs.