Firstly nothing is yet 100% certain relating to Brexit as the exit agreement currently being negotiated has not been finalised and ‘signed off’, nor have any points been agreed about the nature of the future UK/EU ‘relationship’.
If your long-term wish is to be full-time residents of France then it is very likely to be simpler to move over before Brexit (March 2019) or at least before the end of the planned transitional period at the end of 2020. After that then you would probably face tougher ‘third country citizen’ requirements in order to have permission to live full-time in France, which would probably include a visa application.
Carte de séjour applications only concern full-time residents in France so not you at this stage.
If you decide not to establish yourselves as residents, then as a third country citizen who is visiting it is expected that you would be restricted to visits of no more than three months in any 180-day period after Brexit.
Third country visitors to France will also, by 2021, have to apply online for prior permission to visit under a scheme known as ‘Etias.’
An Etias permission to visit the EU will cost €7 and last three years or until expiry of the passport registered when the applicant obtained it, according to the latest proposals.
Note: We cover these topics extensively in our new Brexit and Britons in France helpguide available at our website connexionfrance.com or in selected newsagents across France, priced €12.50 (plus P&P).