Hi Marie,
For the two trips outside of the city:
Versailles,
Euro Disney, it would make sense to buy aller-rétour (round trip, i.e. two one way tickets)
Billet Ile-de-France tickets for those trips and use a 3 zone 3 day
Paris Visite within Paris for the rest of the time. (As opposed to a 6 zone 5 day Paris Visite, priced at
48,40€/adult, 24,20€/child.) This would save you roughly 20€/adult, 10€/child, or 70€ if you consider your party as 2 adults and 3 children (note that RATP considers children 12 and older "adults" in terms of fares).
The downside is having to manage a bunch of single tickets, which ones are which, which ones are used, which ones are unused.
If you decide to go with the mix of Billet Ile-de-France et the Paris Visite, it should be pretty easy to do so at any ticket window or at the CDG ticket office: Versailles & Euro Disney are very popular destinations. Note that you can buy these tickets (including the Paris Visite) all in advance and use them whenever you like; they don't have to be used on the day of purchase.
Hope this helps,
Ben