I agree that parents are and should be the main source of support. Nonetheless, child psychologists can be of help particularly in some situations.

Take the case of adoptive parents, for example. Often the adoptees are children that come from abusive and dysfunctional homes, which imply that they have a high chance of development some pathologies if don't have them yet actually. A parent, even if is one of the most warm, attentive and responsible, probably would have some problems dealing with a child who suffer from any patology or historial of abuse unless prepared in child psychology at a professional level. That is why (sometimes, at least) the close observation and orientation from a professional can be of a great help, if not necessary, for the normative development of the child