They died happy (spoilers)
The film was very depressing, but it didn't present suicide in a very bad light, in my opinion. This man and woman died happy (I'm not sure about the daughter). This was a "happy suicide", so to speak. They killed themselves because their life couldn't get any better. I'm not saying their life was perfect, but it was reasonably good, and there was little hope it could have got any better. So they decided to "step out".
Most of the suicides are done for completely opposite reasons (I imagine). Mostly, people kill themselves because their life has got so bad that they just don't see any way out. And they don't have anybody to share their problems with. This is a typical "unhappy suicide". In this film we saw a different suicide. The man and the woman really had a shared desire to do it and seemed to understand each other beyond words - they were not alone, they had found their soul mate.
Of course, for people left behind it must have been a shock - why on earth would they kill themselves? And it made the film so much more intriguing.
I do think they shouldn't have taken their daughter with them.