This estimating issue will come up multiple times in elementary school for your kids if our experience is typical. Seems like gifted kids find it an especially useless exercise. And the logic for how to estimate is so fuzzy in their math books (unlike rounding, which is precise and has rules). Drove my D crazy, but fortunately it was short lived and we are past it now. And when she finally was allowed to take algebra, she had a teacher who really recognized her ability to look at math in different ways, and dubbed one of her methods the "our last name" Method for solving one type of equation.
