this is a term of endearment which is actually adult in origin.

Take it as a loving compliment. And remember, the intent counts.

I don't think this is childish at all. This makes me want to know if this is an example of her general behaviour? If she behaves immaturely in the first place then maybe you could say this is another example of childish behaviour. Otherwise, i take this as absolutely normal. My boyfriend and I have similar expressions for one another, including the "Buffalo girls won't you come out tonight..." song. Some might find this odd I suppose.