My experience of Kempervennen is - shock!- 10 years old!!
Took the DSs, 8/10, June half term and it was great. We drove from Hampshire, via a ferry, overnighting, both ways, in Ghent, for a bit of kulcha; but we met a family from Hook at Kempervennen who'd done the tunnel and the drive in one go.
I can't speak for today, but a wonderful thing was the prices didn't spike like they do in the UK, presumably because they had Dutch, Belgian and German school holidays to consider, as well as UK ones.
The swimming pool and rapids thing was great fun; mine also did a skiing lesson (not that successfully....) but, like in the UK, book your bikes early. We also went out to the local town a couple of times for supplies and a meal or two out.
Back then, our accommodation was basic, but clean, but we didn't spend much time there.