Vacation ideas to maintain kids' German skills

We're an American/German family who spent 2 years in Munich, 2 years ago; now my once-fluent 6 year old is losing his German, and we're thinking that next summer we'd like to combine an extended family vacation back (also including a monolingual will-be-3-year-old and a reasonably bilingual 10 year old) with some kind of summer program for the kids, to get them (back) into a German-speaking environment.

The older boys spent some time this past summer in a "day camp" here; but so far as I know this sort of thing doesn't exist in Germany. Maybe a Ferienwohnung with a Miniclub of some kind?-- but without being in Germany and going to a German travel agent, I don't have any good ideas. (Doesn't have to be in Munich specifically.)

Thoughts?

Some of these hotels have self catering options and most have kids activities for different age groups.

http://www.kinderhotels.com/