No sorry! It won't work. From my experience with dd, kids need to be immerced (sp!) in a language where most of their peer group speak it.
Kids just don't care about learning another language. No matter how frustrating it is for adults.
Dd was given Italian lessons at daycare and school since the age of 3 (living in an italian cultural area in aus). 6 years later, she refuses to acknowledge a word of it. I don't believe she doesn't understand or speak any, but she has zero interest. May if she does spanish in the future some may lurking in her brain (maybe!).
Dh comes from a non-english speaking country. She was born in this country. We moved when she was 3. We visited his relatives once a year. She was sent to an after school club to learn this language. Again zero interest. Although she could understand bits.
Now we have moved back to this country. She goes to the local school. She is now fluent after a few months.
I expect others will have tales of their kids becoming fluent in mandarin by going to an after school club. But in my experience, it doesn't work.
Sorry I can't be more positive. Maybe go on holiday to France (or where ever). Put them in a local holiday club while you have cocktails on the beach for two weeks?