Hi B., this is a good question and something I have always struggled with! I have a 3 year old and an 18 month old, and have had to really experiment with them both to see what they are most comfortable in at night. I feel it depends so much on the actual child itself, as my daughter always sweats at night and gets hot very quickly. So, with her, I have learned to dress her fairly cool - short sleeves and long pants. Now, thankfully, she is at the age to know to cover herself with a blanket when cold. My son (18 mos) on the other hand loves to be warm, and sleeps better when in long sleeved top and pants (no socks). He would wake a lot when dressed in less. I am always prone to err on the side of keeping the little ones warm, as they kick off any blankets anyway during the night.
Good luck and you are not alone with this concern!!!