to Mabillon.

subway line C.
Change at St-Michel Notre-Dame station and take subway line subway line 10.

Paris Métro Line 10 is one of 16 metro lines in Paris, France.
The line is entirely underground and stretches 11.7 kilometres (7.3 mi) across 23 stations. It has the least traffic of any of the 14 main metro lines (excluding lines 3bis and 7bis).

We never did make it to Romainville. We understand it's near the French Disneyland.

This is the route we took that involved a switch:
Go to Invalides and take subway line subway line C
towards Saint-Martin d'Etampes, Juvisy ou Dourdan ou Massy Palaiseau, Versailles Chantiers ou Pont de Rungis aéroport d'Orly
1 C Invalides
2 C Musée d'Orsay
3 C St-Michel Notre-Dame
pieton
Correspondance
Change at St-Michel Notre-Dame station and take subway line subway line 10
towards Boulogne Pont de Saint-Cloud
4 10 Cluny La Sorbonne
5 10 Odéon
6 10 Mabillon