We are in France; bilingual family (English mother, French father) but as I am a SAHM and for various other reasons (no French family, lots of contact with English family etc.) both our DC had English as their majority language and only really understood French until they started maternelle.
We heard the same thing: "Just wait until Petite Section ; the French will take off and you'll struggle to keep their English up!" and, endlessly, "by Christmas they wilk be babbling away in French".
Well, no, that's not what happened to Dd1 at all.
She improved steadily throughout PS, but her French was way behind her English and she never, ever spoke French at home, even with her Daddy. Teachers at school had no concerns, luckily, as the are lots of monolingual kids who start PS with not much spoken language. But it felt like slow progress, knowing as we did how good her English was.
In Moyenne Section she got better and better, but still not babbling away / fluent and certainly not better in French than English.
She is now in Grande Section and I would say that her French is pretty good. She had a supply teacher for a week last week and that teacher did not know she was bilingual and had not noticed any shortfall in her French versus her French school friends. She overheard me speaking in English to Dd at the school gate and asked me about it and was shocked as she said Dd's French is absolutely fine. This was certainly welcome news for me! So I would say it has taken nearly 2 and a half years of school to get to age appropriate fluency.
Funnily enough, I still don't think her French is as good as her English, but I now realise that hee English is really very good, not just by bilingual standards but by any standards :).
So, I think thise who bang on about fluent by Christmas either have exceptionally attuned to language children OR do not have bilingual children at all and don't knownwhat they are talking about.