I would definitely not tie the foal and would not separate them. We prefer to use our neighbor’s nice professional rig with the box stalls (he is a prefessional transporter), but second choice is our 2 horse with the divider removed and the doors all closed up (windows with screens open). Our trailer does not have anything they can get hung up on and we do not tie the mare. We leave their leather halters on and have help on both ends of the trip. Thankfully we have never had an issue with mare or foal.
Good luck - as folks have said, prepare well and it will most likely be a non-event.