I have never been to Phantasialand, but when I was a kid I went to Europapark once or twice. And I have to say that I never found it that fascinating. Maybe because i'm not into roller coasters, it simply wasn't my thing.

As mentioned before: I'd prefer a day at the Bodensee (in the summer), in the winter you could visit Alpamare or Säntispark. It's fun for both parents and kids.