My 23 mth old son also the same! Sweats easily especially the head. If he doesn't sleep with air-con, his head will wet the pillow. But his nose quite sensitive, tend to get blocked nose easily. *sigh* So usually I let him wear full body suit to sleep, or if normal pyjamas wear socks.
Could it be body type or genetic? My boy takes after his father and has a 'hot' body, they get hot and sweaty very easily... whereas I hardly sweat even in 40+ degree summers!!
I reckon it is normal. Sweating is a way of cooling the body down. Just dress them accordingly