There’s a reason why retro anime from the ‘80s and ‘90s are still so popular today. The stories are timeless, thanks to their focus on character development and complex storylines.
Based on the good ending in FNAF3, where the souls of the children are finally released. This is just my interpretation of what the child inside Bonnie looks like, and a way to give them back their identity. Because the series really is about the tragedy around these children, but we don't even know what they look like. It's a bittersweet moment, but a hauntingly beautiful one.