Eyes wide open, is the best advice I can offer. Leave any rose-tinted specs at the ferry port. Throw yourself into your new life with enthusiasm, but above all - learn the language to a reasonable degree.
Usually the kids are fine, they're like sponges where new things are concerned and after around a year, you'll notice they're becoming more & more fluent. Don't expect it to be sooner. It still takes time.
I'd move before the school holidays begin here in France Tell you why. It's two whole months of dread and anticipation for all of you. If they have a week or two before, just to get used to their surroundings, and in order to meet other kids, then that (in my opinion) is better.
Live within your means, and make sure you have plenty put away for those little emergencies!
I'm not going to say it's easy, and that it's fantastic. It isn't. It's the same here as it is anywhere else, with the added disadvantage of being a stranger, in a strange land. But, with the right approach, it can be rewarding and you'll experience a way of life that can be better than the UK, but you'll need to work at it.