Karen, very good questions.  My daughter is 14-1/2.  Just last summer she was dehydrated.  She wasn’t careful about drinking enough and it was VERY HOT.  I don’t know that she had actually been outside doing a lot, nevertheless, she was in the hospital for a day with an IV to re-hydrate.  She does sweat a lot in the heat and I know she loses more than a normal amount of salt as a result.  Heat is just about her biggest stressor - bigger than when she has broken bones!  I take the heat very seriously with her.  Even a very crowded, hot, stuffy room can give her problems.  OF course every child is different, but this very definitely affects my daughter.