Child seats and boosters are not mandatory in taxis in France:
Article R412-2 of the "Code de la Route"
(...)
III.-Toutefois, l'utilisation d'un système homologué de retenue pour enfant n'est pas obligatoire :
(...)
3° Pour tout enfant transporté dans un taxi ou dans un véhicule de transport en commun.
Most taxis do not have any. If you have one which matches current regulations you should be able to use it.
Some companies can provide taxis with child seats and/or boosters. For instance, G7 (one of the largest taxi fleets in Paris) offers the G7 Famille service. You can book one via their app (select "G7 Famille" in the list of vehicle types, then the number of baby seats and boosters). You'll probably have to pay a little bit extra (4 euros if you book it on the spot, more if you book in advance, which includes waiting time in case of delays).
Other companies may have similar services.
Note that as soon as you add restrictions/extras on a booking it may increase your wait time, especially at peak hours, and that the pick up spot for booked taxis may be in a different place than the regular taxi queue.