The population is constituted by preschool children in general, as it is the human group whose reaction the teacher wants to assess. In order to achieve her purpouse she uses a sample, the 30 children in her class.
The type of data she collects is called experimental data as she is observing the effects of a treatment (sugar consumption) in a controlled environment conditioned by the needs of the research and she is storing the results in order to draw conclusions from them.