It's a tough one, I agree. If I were going to be teaching children, I would probably say that Prince Gautama loved his wife and little boy very much, but he knew that he had to find the truth about suffering and the way to enlightenment, and he knew he had to do it alone. And I'd probably also mention that Yasodhara and Rasula later joined him, as Bhante mentions above, and that they ultimately attained nibbana themselves -- the highest happiness that can be known.
On a more "grown-up" note, there's one aspect of the story which I don't see discussed much, but which strikes me as possibly important: Gautama, at the time when he left home, wasn't an enlightened Buddha but a confused and tormented young man. Would the family have been happier if he'd just grumbled along, knowing his calling was elsewhere but staying put out of duty?