Divorce and Child Support

it just gets better and better doesn't it????  lol.  my husband and i are quite concerned about his youngest daughter.  She lives with her mother.  Her brother lives with us.  She is 9 and has some issues that the school has talked about special education but no formal diagnosis or anything yet. she only knows how to push one button on the phone, can't tie her own shoes......basically, would not know what to do if there were a true emergency.   her brother has aspergers syndrome and she exhibits some of the same characteristics.  anyway, their mother works various shifts/hours.  my step daughter is in after school programs until 6 p.m.  After that time, her mother will leave her home alone while she goes to work.  when her brother lived there, his mother would have him babysit...even though he's only 2 years older and has a disability.  she even had him stay home from school once to babysit his sister because she was sick that day and couldn't go to school.  he moved in with us shortly after that.  you're seeing my frustration already i bet.  anyway, i'm a mandated reporter with my job and have called dcfs....and they won't take the case.  my husband has tried to talk to his ex wife about our concerns and she states that we've got her only babysitter here and that it's not illegal because she is a latchkey kid.  any suggestions????  what is latchkey?  is there a time limit on that?  age limit?  we're at a loss and worried sick and so is her brother.