I forgot about messageparis! It really is great and you will get loads of support and advice there. You won't get the low-down on schools though -- parents will say only positive things in response to a qn, or not reply at all.
Good Schools Guide covers Paris -- it will give you a very comprehensive picture of the different options for anglophone expats in this area plus, I think, very detailed reports (mentioning any niggles) on all the best schools. That will surely include BSP and Forest as they only have happy parents.
Malherbe parents are divided. It's privately owned and profit-making, which from bitter experience I would always avoid, but that's mostly because in the later years you can get serious problems -- if I were staying for a limited period I would "take a view" on the benefits of learning French properly against any drawbacks with Malherbe (facilities not what you might expect from a fee-paying school, a certain feeling that parents are not listened to...). The head would like to make investments but is probably reined in by the owner who has her beady eyes on the bottom line. There is at least a parents' association but it has not real input other than organising social events... you cannot for example go to them with a grievance and expect results unless it happens to suit the school.
On the other hand don't assume your girls will be behind on reading and writing in English after three years at Malherbe. The bilingual schools pack a lot into their programmes and usually set a lot of homework -- which is where you will come in!
If your worries about bilingual in general or Malherbe in particular override the advantage of learning French, then of course BSP or Forest are your best choices. Forest is very small, very personal, very encouraging and really looks after everyone. They love nature and use the forest across the road as an extension of their classrooms. BSP has fantastic facilities, extra curricular and sports. Relationship with the parent body is great, it is very structured in its approach to everything (pastoral care, nurses, tutors, communication, PTA etc) and you will really feel like you have joined a top fee-paying school in the UK.
Don't stress it too much, and good luck! Any other qns would be happy to have a stab at...