Answer:
The researcher is confounding the sweetness of the cereal and the colour of the cereal.
Explanation:
Based on the results, it would be unclear if the subjects prefer the cereal because of the colour or because of the sweetness --it is only the latter the researcher wants to know about. This would be the dependent variable.
To fix this, the researcher should use the same colour for both sweetened and unsweetened cereal and have each preschooler try both. It is possible to colect data through observation (look at the children's reactions) or simply asking for their opinions at the end of the experiment.