You shouldn't run elective tests/treatments on a foster child (and a temporary placement at that) without permission from the worker. If you're that worried about infectious diseases, institute the proper cleaning/precautions. Have gloves available if you need to treat a scrape and you have an open wound on your hands. HIV is incredibly hard to transmit. You may wish to think about immunizing your family (not your foster child, though it's likely they already have been) for Hep B. Educate your children about best practices for first aid/bodily fluid clean up.
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You may have access to a bloodbourne pathogens class through your agency (if not, they're required for child care workers in many states, so you can find low cost options when that's the case).
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If you need bloodwork on every child coming in to your home, that's really something that you should make sure the workers know about. You do NOT want to do it first and ask permission later, which would probably (rightly so) put your license at risk.
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In any case, operating with common sense precautions is really something you should be doing anyway. As much as this is unfair, unfortunately medical records get screwed up, information is not/mis-communicated, all kinds of weird stuff is going on. It's good to get into the habit of using gloves to clean up blood and body fluids in general (it's not weird or impersonal, though until it becomes routine it might feel like it). And if you do ever take care of a kid who has HIV, chances are a) there's no way you won't know it, because your life will be consumed with the medications (unless things have gotten a lot better than when I last took care of a kid on the meds) and b) YOU and your children are far more dangerous to that child than the other way around. Hep B made me more nervous than HIV, to be honest. But even then--it's unlikely to be a problem unless you regularly swap spit with a carrier or share needles.
 
