I agree with the letter writer. Imagine telling a child that their beloved pet would be eaten by worms after being buried - can you say nightmare fuel, especially for a four year old? There's no real need for that much truth for a very young child.
Instead, tell children that burying their pet will help to fertilize the soil and make grass and flowers grow. This is also true, but it sounds far better than telling them that it will be eaten by worms.