Enjoy the cruise.
I'm looking forward to the Disney Magic from Galveston myself in October. I will be experiencing the ship for the 1st time too.
IMHO--Cozumel has plenty to offer a family without wasting valuable port time on a ferry over to the mainland and back. Especially with a 3 year old and with a boarding time of 3:45 pm. Try one of the many beach clubs or Chankanabb Park.