Check out this web site...
http://www.ctf.state.al.us/. There is a number to call and they will be able to tell you what the rules or laws are for the state of Alabama as far as the age it is appropriate to leave a child alone.
It may come down to the maturity of the child and the feelings of the parents. If you and your ex disagree on the matter and he feels she is not mature enough to be left alone then you may have a problem that means modification of the original decree. I agree that summer camp would not be considered child care...not in my mind anyway. At her age though, if your ex feels she still needs to be with adults while he works then he may feel that putting her in summer camp woudl be better than daycare because she has gotten older.
Is this summer camp more expensive than the usually summer child care that she has been getting? Is there no language in your divorce decree that would cover resolving disputes such as this?