another thing to be aware of is Chol HaMoed trips (or anytime of the year) and stops for rest rooms or gas.
When we stop and kids need the facilities or at the end of the trip and we are getting ready to go home, being as we do not count people directly, we do a sound off. I make each kid confirm that their seat belt has been buckled. I call out in age order (either direction) and wait for a positive response from each before moving on. an older kid will confirm for a young child that the belt is really buckled and the one sitting next to the baby will confirm for the baby.
This helps in 2 ways: 1) nobody is left behind and 2) everybody is safely buckled in.