I'd say I have nothing to add (although I think the response is spot-on), except to say that I think the, "they're just not old enough to understand" thing is utterly ridiculous. Children can at times be very rigid in their understanding of the world, because the over-apply the patterns most familiar to them, but for the most part, they accept what they are presented with as normal. I remember at my sister's wedding, my then-eight-year-old niece (who was the flower girl) earnestly explained that she'd had to be the flower girl at my wedding and her other aunt's, because we didn't have little girls. For her, it was normal that the bride's daughter would be her flower girl, just as it was normal for her that her parents didn't live together.

The kid understanding is really not the problem.