I need to get my in-laws from Paris (19th Arr. / Belleville Metro) to CDG on Saturday morning for a midday flight. They don't speak French and haven't traveled in Europe before, so I'm thinking a taxi or Uber would be better than the RER. Is the taxi fare from Paris to CDG a fixed fare like the taxi to Paris from CDG, or is it different? Is Uber a better option?
Yes, the taxi fare is fixed in both directions.
For your parents going to CDG, it would be 50€ PLUS:
4€ if you call and reserve the taxi rather than getting it at a taxi stand or flagging it; OR
7€ if you call say the night before to reserve the taxi.
Depending on traffic, an Uber can be cheaper than the fixed taxi rate — but it can also be more expensive. It depends on surge pricing, etc. But traveling from Belleville, you're already at an advantage (I.e. the possibility for the Uber to be cheaper) because you're in the NE corner of the city as your starting point, which helps.
slight tweak -- it is 7 Euro to reserve a taxi for a time regardless of whether it is 3 hours from now or the day before. It is 4 Euro if you call one for use right now. 50 should be placed on the meter when they get in the cab -- the reserve fee is then added on at arrival usually.
Thanks for the clarification Janet!