We live in the UK, but speak French at home as my husband and I met in France (and in French). Our daughter goes to the nursery full time, and therefore the switch between French and English is very clear for her, but she is only two and in the process of acquiring so much vocabulary in both languages that of course, she mixes everything altogether and comes up with wonderful Frenglish sentences. My husband and I anyway do the same as some English words and their exact meaning are sometimes more appropriate in our French conversations...
Two things we noticed however: although our daughter mixes words, if we start telling her English words, she corrects us with the French word. And she has been doing that very early (around one and a half?), so although she is mixing languages when she speaks, we suspect she has a clear idea of what language she is supposed to use (again, boundaries are probably clearer for her, who speaks only English at the nursery and only French at home, than for children who speak several languages at home).
Also, some friends' daughter, French as well but going (part time)to the nursery in the UK as well, has been speking English to our daughter naturally, even when our daughter was babbling more that she was saying understandable words (although everybody speaks French around when we meet), whereas our friends say she would speak French to another friend of hers (I believe she goes part time to the nursery as well). We are not sure whether our daughter started speaking English first to her, or whether her babbling just sounds English more than French... anyway, all very interesting to watch...