Do yourself and your kids a favor by putting them in a regular German school. It's a lot cheaper, and your kids will be fluent speakers within a year.

I know this first hand. I was placed directly into a school in Stuttgart with only German speakers for my 6th grade year, and left a year later fluent in German.

The only thing my aunt did "extra" was hire an occasional German tutor...

So go for full immersion - culture and language - for your kids if you're lucky enough to get based in Cologne.