I am currently living in France with French DH and our 4 DC, aged 11, 7, 5 and nearly 1 year old.
We do OPOL, but having grown up in France their whole lives, our DC have much better written French than English. We've always had English tutoring to give formal lessons, but school days are long in France and they have a lot of homework to be getting through, without the added burden of extra English on top.
For those of you who live outside of an English speaking country, how well do your DC speak English? And how do you cultivate a decent level?