With regards to older children, one method I have seen used to great effect is CLIL, content and language integrated learning. The focus of the lesson is the content, and the children have a need for the language to learn the content, hence they are motivated and don't realise the hard work.
Where I live in North Spain the govt has passed a law stating that children must learn some school subjects in English. It varies from school to school, but I know children who learn maths in English, or science (very helpful as if you go on to do science in later life it's all in english).
My subject is English language but I've experimented a lot with this idea, and the best lessons are the ones where the focus is the content. We learnt about the titanic and through that, the third conditional, but the kids NEEDED the third conditional so they could imagine what they would have done differently if they were various people on the titanic.
One poster mentioned immersion wrt films, TV etc. this is totally possible in the UK too, just look at the back of all your DVDs for the available languages. For younger children, you can get disney sing along songs in French and Spanish (at least, probably more) which I was obsessed with as a young child and can still remember the words to the Disney songs in foreign!