Practical considerations
It may be preferable to have observation done covertly and if so, careful consideration must be given to how this is done. Children in school dining halls are accustomed to being supervised by adults and may not perceive the presence of a researcher as unusual.
One study observed children in school and in their home over a year to establish familiarity without collecting data. The following year they then colleted 3-day observations from 40 children (Davidson et al, 1986).