Welcome Vera! Your daughter and grandaughter has a wonderful support system, that being YOU!
One of the main reasons I had my son tested was the salty skin/sweat. He would have salt crystals all in his eyebrows and sideburns when it was just a tad warm out. He also had failure to thrive. He is doing very well now, although he could pack on some more pounds.
Ask any questions and there will be someone here to help you as best as they can.
Good luck and hugs!