We went to lunch with some friends today. They are both Russian, live in London and have two children. They all speak Russian so it was great for our children's Russian to have an afternoon of playing in the language: although we have a few Russian friends, very few of them have children who won't automatically slip into English with other children (and many have children who don't speak Russian at all).
What was interesting was that they make a particular point about speaking Russian to their children: they forbid the use of English at home and also constantly remind each other to stick to Russian as otherwise they find that they slip into English. So even when everyone in the family speaks another language to start with, English creeps in and seems to gradually take over!
This highlights the challenge for bilingual and trilingual families: if English (or whatever the community language might be) tends to dominate when both parents speak the same other language, this is even more the case for families where each additional language is only spoken by one parent. So well done to all who manage to keep multiple languages going!