One person did "try" to answer the question.

Hello oberost,
unfortunately I haven't stayed at the BW Marais-Bastille and, strangely enough, there isn't s single review at tripadvisor either.

But I LOVED the Best Western property in the 6th: Left Bank Saint-Germain.
18th century house, perfect location, tapestries, exposed beams and a good breakfast included.

The Marais Bastille's rooms lack some charm in my opinion while the lobby looks fine. And the location seems to be just right.
So if the Left Bank Saint-Germain happens to have rates as reasonable as the Marais-Bastille I recommend you switch hotels...

Otherwise, the Marais-Bastille hotel will become a part of a really great trip for your granddaughter: She's very fortunate to have grandparents like you and your wife!