Can children form an impression from a face?

Yes, they can, and they do early in life. There is research in progress going on right now, to address exactly this question, especially on how early they develop this unconscious cognitive ability.
A research study supported by the E.J. SafraCenter for the Study of Ethics, the Santa Fe Institute, and the National Institutes of Health called “Inferring Character From Faces: A Developmental Study” shows that the predisposition to judge others based on physical features starts early in childhood and does not require years of social experience. Children evaluate and judge faces of other people like “which one of the people is very nice”; in fact, they showed consistent ability with that of adults to judge especially trustworthiness (i.e., mean/nice), and then dominance (i.e., strong/not strong), and competence (i.e., smart/not smart). [..] „The study also suggested that children may tend to pay particular attention to the demeanour of a face – that is, whether it is broadly positive or negative. Importantly, the findings do not address the question of whether the judgments the children are making are accurate inferences of character. Rather, they simply demonstrate that adults and children are consistent in the traits they attribute to faces, irrespective of the validity of those judgments.” (source: https://www.healthcanal.com/mental-health-behavior/48291-young-children-form-first-impressions-from-faces.html)