As everyone above, my daughter now 3 Yrs. also sweats a lot.  Endo. says it’s not anything but her body make up.  About the salt--at 3 she can tell me when she wants/needs salt, but 2 years ago I would just make sure she didn’t get to hot, always going in in the air.  Also, I have left a window fan running (pointed the other direction) during the night since she was 3 months.