Actually, preboarding ONLY makes sense if you have a car seat. Even then, if there are two adults, it really only makes sense for one adult to go on ahead and install the car seats and the other to wait til the very last second to go on. There is absolutely no need for a kids to be cooped up on a plane longer than absolutely necessary. It really does not take much time at all to buckle a seat belt and stow a backpack under a seat.
Have a fun flight