I grew up bilingual, so I'm clear that it can work, but I wonder whether you prepared to persevere when it gets difficult, because the devil really is in the detail.
When the children first meet her, how will she be introduced? You (and they) will be speaking English. How do you want her to join in? By speaking German from the start? How will you know what she is saying? Its not easy for people to communicate unless they share a common language. When does she switch over to German? When they are alone and she is new and unfamiliar ti them?
Once they get going, how strict do you want her to be? If they are feeling unwell, or are in pain, are they allowed to express this in English and have her understand, or must she insist on the other language? Once they understand that she understands their English, how strict must she be in sticking to speaking in German? What language will she speak when you are all together as a family?
Are you planning any visits to bring the language alive? There are german bakers in London, and Windsor, how about some visits so they can try German food. It would be good for them to see some context for the language.
I don't think my parents put any effort i to it,they spoke on language to me and I 'picked up' the language of the country where we were living. It all just happened. Im sure it is possible to recreate the experience of total immersion but it would be good for you to be prepared for the pitfalls. All the best.