If you're going to teach in the TL, you need every trick in the book, including a 'routine' for those times when you absolutely cannot speak in the TL and only English will do. For these such moments I have one of these:
Child desperately needs to speak to you in English. You will only allow them to if they ask in the TL if they can (and sometimes, I make them state a reason why, like 'because it is too difficult' etc). When they have asked in beautiful French or German (language given to them on the language mats on a previous post), they then wear the above hat. Whilst the hat is on and ONLY whilst the hat is on you are both allowed to speak to each other in English. The SECOND it comes off, you are back in the TL. This can work for you too. YOU must ask for permission to speak in English (Sometimes they refuse you permission!) and on their approval, you wear the hat and the same situation applies. Simple, but effective.
I love my hat!