Its only a problem if each child has a separate car seat. There are companies that produce seats for up to 4 children that fit in the back of the car.

Example from a Google search (don't know if they are good)

https://www.multimac.com/

1. Those are often semi-permanent installs

2. Those must be professionally installed

3. Those are pricey

4. Those are, quite by accident, illegal in North America