Hi Fiona,
Congratulations on your first post, don't worry, we don't bite :-) I have 2 kids, a little bit older than yours who were born here and we have ups and downs with the language but it works out in the end. One of my friends has the opposite problem, her 5yr old(born here) will still not speak French at school after 3 years - his little sister, started last year and she wont shut up, so now the boy is having to visit the psychalogue every week - you just can't win.
Our kids go to the local private school but they are sill the only English speakers in kids so the school will not necessarily resolve the issue. We work on it at home, you don't say if their dad speaks English or not but that will make a difference. We are both English speakers with fluent French and many French/bilingual friends but we make a point of only speaking English at home, we have English TV, English books and English DVDS. When the children speak in French, we answer in English and we also correct the English without making a big deal of it. IT's not that we're obsessive about it, it's just that we appreciate how lucky they are to have the gift of being bi-lingual and we are determined that they should profit from it. Having read that, I sometimes wonder how good our English is these days :-)
Do you live near Abbatial, (no idea where that is, just know it is in the Midi Pyrenees) I have friends there who have children aged 5 and 3 so could put you in touch with them if you like, as that is probably the best way to encourage their English. We have also been sending our daughter to the UK since she was 3, to stay with her grandparents, so she had no choice but to speak English - not as harsh as it sounds, she loves going. In fact, we have misgivings about sending our 5 yr old with her this year as they do tend to play in French - on the other hand, 2 weeks alone, without children for the first time in 8 years----