Is it unlawful for an adult and a 3 year old child to be passengers in a semi-truck ?

Question

what is d.o.t laws on this if there are any laws on this matter

Answer 1

I can't find anything on this in an old Federal motor carriers guide, but I would think that car seat rules would still apply.  Also if you are a company driver you will have to abide by their passenger rules also.  A lot of companies have an age requirement of passengers.  You could probably call your state DOT office or testing station for CDL and find out the answer to this question.

Answer 2

As long as their is a seatbelt for everybody involved, I wouldn't see why it would be illegal.