I can see both sides. But DL sticks to their policy one this one point (if only they stuck to all their policies consistently). Splitting up a family with children is a very bad idea.

BTW to those who don't like kids in BE: I bring mine, I pay for it, and I know how to keep them from disrupting you. If you don't like it, fly an airline that doesn't allow it, if there is one (Virgin?).