A while back a buddy of mine was venting about this girl he was dating. "I don't know man," he said, "sometimes she acts like she's a girl!"
I raised an eyebrow. "I don't need to know about your sex life, dude."
"Not like that," he shook his head. "She doesn't call me 'daddy' or anything. It's just sometimes, when she's excited or happy, she acts like a little girl, singing silly rhymes or talking funny or squealing. It's kinda weird and just a bit annoying."
I said to him, "Does she do that when other people are around?" He thought about it for a second and shook his head, saying he wasn't aware that she did. "Then it means she really trusts you and feels comfortable around you." Now it was his turn to raise an eyebrow. I continued explaining. "Women -especially in this modern age- are forced to grow up much too early, and so very few of them get to truly enjoy their childhood. So, if she's letting out her inner little girl when it's just the two of you, it means she feels safe with you. Don't be annoyed, take it as a compliment."
He thought about this for a second. "But what if I think it's really annoying?"
I sighed. "Yes, you could talk to her, tell her you think it's annoying, but unless she's doing it deliberately to aggravate you, all you'll do is make her feel self-conscious which will not make her feel safe around you which will inevitably lead to your break-up. If that's what you want, go right ahead; but, I'm pretty sure I'm right."
Well, sure enough he talked to her, and they did eventually break up, and odds are she kept that part of herself buried in the next couple relationships which is truly a shame. So, I think if a grown woman feels that comfortable around you to let out her inner little girl, you should be honored that she feels safe enough around you to be playful and whimsical. There's so little of that left in the world nowadays...