Can "My girl" also be used to refer to my daughter?

Without context, my girl is ambiguous. For instance, it used to be a common way for executives to refer to their secretaries (watch "Mad Men" and you'll probably hear Don Draper use the term that way). There's the classic Temptations song "My Girl", which was about the songwriter's wife. And in the Eddie Murphy song "Party All the Time" the main lyric uses the phrase to refer to the singer's girlfriend.

But once you mention the girl's age, it's unlikely to be misunderstood. A 5-month-old can't be a wife, girlfriend, or assistant, so it must be a daughter.

And I think the plural my girls would usually be understood to mean your daughters, unless you're in a specific context where there's some other expectation. For instance, the owner/manager/coach of a women's sports team might refer to the team this way, although it would probably be considered demeaning these days. Or a teacher in an all-girl school could refer to their students this way.

A clearer term might be "my little girl".