DH is German and I am Scottish so we each spoke our own languages, when the DC were born we lived in Germany so they spoke mainly German. Our home language was and still is German.
We moved to Switzerland 2 years ago and the children went to the local school, where they learned French. Neither DH nor I speak very good French, so we hardly speak it at home.
The DC now speak mainly English, as most of their friends here are expat Brits and fluent French and German.
Yes, agree that you have to speak reasonable German for it to be worthwhile teaching it to your child. My German is very fluent, but the DC still sometimes correct me.
Sounds like you have a good plan. My advice is to stick with it, even when others tell you that is not a great idea. You get two types of comments - either "oh, that must be confusing for the child, I hope he/she can cope" or "oh, how fab, they are little sponges, at that age".
Neither is quite right, as it is not quite as easy. Particularly with a minority language, ie. Dutch in your case if you and your child are the only ones speaking it. It is a big commitment you are making, but it is such a great thing to do.
I do think that my DC learned French easier because they were already bilingual.