Yes but seaports like airports are subject to tight security and they will not get past check in they will have have to go to the the viewing area just outside the building .
The best place to see the ships sail is Mayflower park.
There is a visitors viewing area in the Mayflower terminal alongside the ship.
Depending on where they live in the area there are several places they will be able to see the ship sail. royal Victoria Country Park at netley....Hythe esplanade opposite the docks...or further afield Lee on the Solent sea front or Stokes Bay seafront about an hour and a half after sailing (this is where my son goes to see us sail out.
Yes not to sure if you can get in the terminal if not tell them to go to the end of the terminal building and onto the quay side they will have afantastic view of the shi.