I have two daughters who were both born in Spain. We now live in Scotland. My husband is English. We are happy in general with how they are progressing with their two languages, but it is not straightforward. You have to keep them going with cartoons, books, music, etc in the weaker language (Spanish for us now) and regular time with the family in Spain. We have also been taking them to a Saturday Spanish school here in Edinburgh which is amazing. Lots and lots of Spanish children, good teachers and fun classes. The biggest thing is that they want to speak Spanish with the friends they make there and as a result they improve quickly and painlessly. Kids pick up languages really easily and, as someone else in this thread commented, they understand well but can be lazy to speak. The best way for them and for us is if the whole process is natural and organic and they actually want to speak simply because they want to. Finding friends and fun situations on a regular basis is vital. Here are a few other good tips and a bit more on the topic.