FYI...a car seat/booster seat is not required by law for either of your children -- "just" a seat belt (although you may opt for the extra protection anyway):
Amazing! I run an annual "Car seat clinic" with our local public health department. And it amazes me every year how people can think a small child can sit in a sit belt that has been designed for the average size adult. If a child is wearing an adult belt (yes even with the "seat-belt fixers" that are supposed to resize the belt) the chance of a child surviving are much smaller as the injuries tend to be great to internal organs. For the adult passenger the lap belt falls over your hips (large bones at your pelvis will sustain the impact but protect your organs), for a child the lap belt tends to cut them slightly around the waist where there is no further protection and damage to their organs can kill them. You have to wonder sometimes what these lawmakers are thinking?!