Usability testing has traditionally been conducted in dedicated laboratory. Alternatively, field study can be an option to conduct the usability testing. As HCI researchers who work with children, we have experiences in conducting usability tests in both natural and laboratory environments. We tested two different children applications in two different environments: testing drawing applications at their preschool and testing an educational game in our usability laboratory. Thus, this paper highlights our experiences in conducting both usability tests. Although the literature suggests that laboratory and field studies should be done for two different purposes, our experiences have told us that we can actually integrate the steps of field studies into the guidelines for lab tests. These steps are particularly useful for the novice designers and researchers who have never done usability evaluations with children before.