It is important to observe children with objectivity for two major reasons. First, all children should be evaluated using the same scale, no matter what is being observed. Secondly, in many cases the observer may be a parent or other relative who could have an existing bias toward the child.
Observation of anything, children or otherwise, needs to be done objectively to ensure accurate conclusions. If subjective or biased observations are made, the results of that observation are likely to be void and inaccurate.
Whatever judgement is to be made, each individual being observed needs to be done on a concrete scale with meaningful ratings or scores. This prevents the potential problem of "comparing apples to oranges." It also prevents any bias in the observation that may come as a result of preconceived notions about the child. These notions most often come from a relative or friend of the child being observed, but could also be based in other external factors. For example, the observer may know an older sibling of the child that behaved in a certain way and expect the same from the younger.
Finally, objective information gathered in the observation can establish a baseline for whatever attitude, behavior or action is observed. For example, if the child is observed for behavioral modification purposes, establishing an objective baseline can determine the levels of future improvement and success.