In a child's room, what matters isn't how much light there is, but where it comes from. Downlight placement, the color of evening light, night light brightness and how the wall plays its part.
The wrong question is "which color?"; the right one is "how saturated?" Muted tones calm, saturated tones stimulate, and the dose depends on what the space is for.
Well-known characters arrive with their story already written; the wall is the one big surface where a child can build a story of their own. That's why we only draw our own characters.