There's a developmental aspect to it that every child goes through, so that type of idealization is cognitive. In the sense that children aren't capable of processing complex emotions until older and the brain is not fully formed until age 20 something, they also don't have an adult sense of self. It's similar to how a child can't distinguish the mother from self as an infant, followed by being the center of the universe. I think at the point where the idealization begins is when a child discovers the parent is separate from them and lasts to the separation-individuation phase.

It is used as a defense for some clients, in others as you said, projections, but every child goes through that phase and some of experience unthawing those frozen parts in therapy.