The earlier the better. I know a few bilingual couples, they make an effort to speak both languages equally (each parent speaks their own language to the child), so the children sort of learn both languages at once.
My eldest got an hour of French a week at nursery - he could say a few words, ask for ice cream, but I think he's forgotten it all now. If they never get the chance to practice/use, they'll just lose it. He did know enough to sometime work out what we were talking about when we spoke (bad) French so he didn't know what we were saying
P1s at Robslee are definitely getting some French.