alternate ending (?)

You're both right as there were two endings. The "Holocaust 2000" cover version ending is where Kirk Douglas' character is handed his child. We then see a view from above them and the shot gets wider and wider. Meanwhile we can hear the chant "What do we want our children to be when they grow up? Alive!!" We then see a group of people dancing around a bit and the film ends. Sadly that left us with a serious loose end - what became of the nuclear plant project because his son was left running the show.

The other version in my opinion ties things up better. "The Chosen" cover version proceeds more or less the same up to when Kirk's character is holding his child. But then we hear what he hears - voices in his head from a character in the film telling him that his son must be destroyed - or something like that. He takes a flight back to the location of his son (who's now in charge) and has acquired some dynamite which is in sewn inside his jacket (How it remained undetected at the airport is not revealed). As I recall his son thinks his father is dead incidentally. Then comes the couple of shots a previous poster was talking about. Kirk enters the boardroom and his 'evil' son looks up at him. Next comes the explosion depicted by the window shot and presumably they're all dead after that (except for Kirk's new wife and child). I shouldn't really be saying it's Kirk's child but you know what I mean - just the character Kirk Douglas is playing.