How do you casually address a child in the Midwestern dialect?
I'm a male in my mid-forties -- born, bred, and still living in Minnesota. I believe that qualifies as "Midwestern".
When addressing young girls, I typically use one of "sweety", "hon", "honey", "sweet", or their name or nickname (if I know either). Not infrequently I'll use a combination (e.g., "Haley sweet - could you please pick up those wrappers and throw them away?").
When addressing you buys other than my son, I'll typically use one of "bud", "dude", "buddy", their nickname, or their name (assuming I know either). Occasionally, if they're very young, I'll use "sweety" or "hon" with boys, as well.
With the exception of "dude", I hear these same terms frequently from other adults here in Minnesota, as well as in Wisconsin and North Dakota.
My son is a special case. In addition to the terms I use for addressing other young boys, I'll also use the more sugary terms I use with girls, as well as some of the special nicknames he's picked up over the years. I'm careful not to use the "sweeter" terms around other kids, of course.
If the child is a boy, "son" or "junior" is common here in the south. I can't think of much for girls other than the pet names you mentioned.