I try to convince myself that being physically a child, her brain is still developing so even if she has informations of her adult life there are still structural limitations to the human brain in early years so she physically can't think like an adult.
This is looking at it in a realistic way but this is a fictional story and we don't have such things as isekai irl so this is just how i try to make it less weird for me

She’s an adult basically… special forces and shit but she’s so childish?! I don’t mind the baby talk lol. Physically she can’t help it but mentally she’s an adult. I don’t get why she keeps acting so childish. I can’t think of the right word