Languages need a lot of usage over the years if you want fluency. Simply having one person speak some words of German (very simple commands) won't get them fluent in German.
Even families where they do OPOL can find it hard to get that level of fluency, it's hard work!
I know lots of people whose children 'spoke' 3 languages aged 3, by which they meant they could ask for or get orange juice in these languages, but few have gone on to attain any fluency unless it was a parent who really strictly stuck to OPOL, or did extra lessons, or had schooling in that language, or spent a lot of time in that country in the holidays.
It isn't as simple as 'speak the language for a few months and it sticks for life', it requires a real dedication and I don't think requiring someone else completely to be that dedicated is fair.
If you are happy for them to learn colours, numbers and a few phrases, it's entirely doable, but to get them fluent, they'd need exposure to German all the time from the nanny (so no English at all, otherwise they just end up replying in English) and other forms for many years.
Just be realistic. I think knowing words in a different language is a great thing anyway!