I think this is a great idea but you are going to have to be very determined and consistent.
I agree with MIFLAW.
Only French from now on. Speak French, read French, listen to French CDs, radio, spoken word DCs in the car, tune in you internet radio to French stations, French TV.
You can order loads of French books from Amazon.co.uk or from Amazon.fr for a wider selection and slightly higher postage. Your local library might have a French section in the children's library.
You need to make French completely normal for you, normal for your DC.
I speak our minority language (which happens to be English, we live in France) at all times, to the point that the DC regularly express surprise when they hear me speaking French (in shops, at the doctors). My DC did not even know French telly existed until they started school and went to their friends' houses as we only have English DVDs in our home. (That has caused some raised eyebrows when French kids come round and there is actually no DVD they can understand. Not one.)
I read somewhere that the best situation for raising bilingual children is where there is:
-- Quantity (of exposure to the minority language)
-- Consistency (don't switch, don't mix. Even though there are some MNers like Cory who have successfully raised bilingual kids without a consistent language pattn, in your situation, I would not recommend anything but consistency)
-- Need. This is the most difficult one. Your kids can be exposed to French a lot and consistently but if they know on a very deep level that you speak perfect English, that their grandparents speak perfect English, tha they never need to speak French for essential commnication, then they may end up passive bilinguals (which is not the end of the world, still a pretty good outcome, but maybe not your objective). Creating "need" is tricky. But regular trips to France, French friends and family can help.
"Need" can also be falsely created by sneaky parents who offer unlimited telly watching / NINTENDO DS usage / Pokemon cards / whatever floats your DCs boat.....as long as it is in the minority language. I know of a Chinese mother in France who buys her son unlimited Mandarin Pokemon cards. But has never bought a French one. They need to speak / read / understand the language in order to access a pastime of their choosing. Sneaky, but effective.
My DC watch quite a lot of telly, but it is all DVDs of Sense and Sensibility, Little Women, The Secret Garden etc. They don't just get 2 hours of mindless claptrap like Mr. Men or Wibbly Pig. (Disclaimer: I am not that wicked. I do let them watch Fifi and the Flowertots and Maisie, but interspersed with language-rich programmes, too).
My mum (mid 70s) panicked a bit when we had DD1 because she was scared she would never be able to communicate with her (she speaks no French and she assumed DD1 would speak French first and more fluently than English.) so she decided to start learning French. I very nicely told her to cut it out! We are very close to my mum and I knew that a fab way to create "need" is to have a close relative who genuinely does not speak the majority language. The DCs skype Grandma everry Sunday afternoon and see her every holiday.
Bit of an essay there. Hope it makes some sense.
Good luck.
PS you will almost certainly need to supplement with a formal French programme to achieve true bilingualusm. There are many posts about this. You're lucky French is your minority language as there are lots of opps to do formal learning in French, unlike, say, Icelandic or Russian.