I am going to pass four days in France (Disney and one day in Paris) and I would like to know what are the discounts in transports you have for children (I think Navigo Decouvert does not have discounts for children).
Children under 10 can use reduced fare tickets (half price) for single use RER train tickets, 7.60€ per adult, one-way from Disneyland to Paris and 3.80€ per child under 10 one-way. Chlldren under 4 who do not take up a seat are free of charge.
You are correct about Navigo, which does not have discounts for children.
Child discounts for multiple day passes (Paris Visite) are available. See a list of Paris Visite Prices.