Does a baby pay for Roissybus? I understand the child fare is no longer available, but how about a baby?
The transport authority website states that children under 4 travel free on all buses. (I can't find the information in the anemic part of the site that's translated into English.) I don't see any mention of an exception for the airport buses (Roissybus, Orlybus).
Children under 10 get a half-rate only in certain (common) conditions: when you buy a book of 10 “ordinary” (“t+”) tickets, you can buy children tickets for half the price; and suburban (point-to-point) train tickets are half-price for children. There is no child rate on the airport buses or Orlyval, so children aged 4 and up pay the same price as an adult.
The Parisinfo site mentions that children under 4 travel free. It isn't clear from the typesetting whether that applies only to Orlyval or also to Roissybus and Orlybus, but as far as I'm aware it applies everywhere in the publicly-run train and bus network around Paris. (The English version has no mention of this, and outdated rates. Sigh)
The price guide from the transit authority for the Paris region also states that children under 4 travel free “on the whole network”. The RATP detailed rules mentions that as well (though the language does not clearly state that they always travel free), with the stipulation that this does not entitle them to a separate seat (but then paying adults aren't entitled to a seat either, just to transportation).
Note that these rules do not apply to private transportation that is not part of the transit network, such as the Air France coaches (free for babies only up to 2 years old, half-price up to 11 y.o., and with a slight reduction under 25). Air France coaches are significantly more expensive than the RATP buses except for large groups of children between 4 and 11, but a bit more comfortable in that you're less likely to end up in a crowded bus with barely any standing room (though in Paris, if you have a young child, feel free to ask for a seat, people might not volunteer to give up their seat but most will oblige).
You might be out of luck. Although the RATP, Aéroport de Paris and Paris Info websites mention different prices for the ticket (11
EUR the first two and 11.60
EUR the latter), they only mention one price. Moreover, the Paris Info website says that there is a unique fare (
tarif unique in French):
Roissybus est une ligne de bus sans arrêt entre le centre de Paris et l'aéroport Roissy Charles de Gaulle à un tarif unique 11€60.
It would therefore seem that the Roissybus ticket is one-fare-fits-all, without distinction between adults, children and babies.