nowadays, most trucking companies have an age limit on the passengers, and number of passengers due to insurance regulations. Generally speaking, drivers are only allowed to have 1 passenger/ridealong at a time. from what I have seen, usually the age limit for children is 6 and over (I was a lease operater at CR England and that was their min age, for company drivers, it was 12 yrs. age req.)
When i first started driving, (this was over a decade ago mind you)
me and my husband drove a truck for his brother. I was able to drive the truck up until 4 weeks of my due date during my pregnancy, and took my newborn daughter back out on the road with me when she was 2 weeks. I drove the entire time, although it was mostly just as a backup to my ex-husband after we brought our daughter along.
We encounterd all different sorts of problems during the time that she was with us on the truck. Several shippers and recievers had insurance regulations, even way back then, that would not alllow anyone on the premises under the age of 12 (even if they were inside the truck the entire time)
you also should consider other very real day to day occurances.
things like bathroom issues... Sleep issues.... playtime issues,
what would you do if you couldnt be allowed to be in the truck while being loaded or unloaded, but yet children werent allowed inside dock areas.
what would happen if the child distracted his father while driving, and caused an accident, or near accident.
It could even be something like my nephew did while riding with his dad, at age 4, he woke up in middle of night, opened truck door, and fell out, breaking his arm. No hospitals around.
Rules and regulations are consistantly becoming more stringent for drivers, and for passengers.
the road is no place for a family.