The reason most of us end up with multiple boards is because none of them are good for everything.
You will have plenty of fun with either the small or medium Rio. They are the best suited for learning and progressing for your partner and children. As you get more confident, invest in bigger sails and when you are ready then look at an extra board like the Go or other more fancy models. You can then be sailing fast while the others are using the Rio instead of sitting on the beach. I know everyone has budget constraints and has to make choices. If you are more concerned with the whole family fun then the larger Rio is your best bet.