When debating on whether it is practical, per say, to tell a child that heaven exists or not, we must first look at the issue on a grander scale.
At youth a child is pure, innocent, and full of joy and curiosity about the world. Now, imagine taking that joy, stomping on it, then throwing it down the gutter - an innocent whose hopes and dreams were crushed because of the necessity to be "rational" and "realistic" about the world? Does this seem healthy for that child? Does this seem like a nice thing to do to an innocent youth who has curiosity about the cosmos and its mechanics? No, it doesn't. It seems like a dick move, pardon the French. For someone who advocates rationality, it is not worth crushing a child's joy for the terrestrial ball.
Now, one might argue, "Well, if they never hear of the concept of heaven, they'll never be crushed." This is not so much about heaven as it is death. Imagine telling a child that after you die, that's it. Everything you know and love stops. For good. End of story. A child is not meant for these ideas. A child is not meant to learn about a soul-crushing, heart-squishing, joy-shattering "reality" like that. It can bring about enormous psychological detriment. And if keeping a child's happiness is worth lying to that child, so be it.