We are a family of 6 (4 adults, 2 kids under 12) arriving in Paris on a Saturday. We are staying near Disney. I had a look at the various ticket options as we will be travelling into Paris 3 times during our stay as well as Versailles. I am looking at the Ticket Mobilis for the Sunday when we will be travelling to Versailles and then the Navigo Decouverte from the Monday. We will leave again on the Saturday. Is this the best option?
Also, can we buy these tickets at CDG when we land?
Your research on ticket options was well done. The Ticket Mobilis for Sunday + Navigo Decouverte week pass for Monday - Saturday is a good & likely cheapest option for you and the family. Unfortunately there's no Navigo discount for the kids, but it's still the best solution for convenience and coverage.
You can purchase both the Mobilis and Navigo Decouverte at CDG Airport train station ticket offices. As an option, since you'll have Navigo cards, you can put Navigo Day Passes onto your Navigo card, 1-5 zones, valid for Sunday, rather than using Ticket Mobilis. Price is exactly the same, as is the coverage and you get the convenience of having only one fare device, the Navigo card. (Ticket Mobilis will likely be retired in the future, due to overlapping coverage).
For your initial journey from CDG to Disney area hotel, with all your luggage, a taxi van seating 6 may be your most convenient (and similarly priced) option. (For reference RER B + RER A train ticket cost for CDG to Disney area is about 16.60€ per adult and 10.30€ per child.) Here's the official CDG website page with a link to a partnered taxi company (G7) where you can reserve a taxi van.