I will be arriving on Friday evening to Orly airport with my 13 years old son. We will be 2 days in Paris center, than we will go to Disneyland for 4 days and from there back to Orly airport.
What will be the best costs option for us?
At Orly airport, buy 2 Orlybus tickets & 2 Navigo Easy cards each with a carnet of tickets. This will cover you for Orly - Paris and 2 days in Paris. Then buy one-way RER tickets for Paris Disneyland train.
When returning to Orly from Disneyland you can buy RER tickets to Denfert Rochereau from Disneyland and then take Orlybus back to Orly. You can buy both RER and Orlybus tickets from Disneyland train station.
A child of 13 years old is considered an adult, so no discounts are available.