I went with a size Large in these pants for my 9 year old son, who is 5ft, 70lbs. They are quite roomy, but in time he will fill them out. He's pretty slim, and these do seem like they would fit someone of an average build better, but they are very soft and have side pockets. The only issue I see with them is the material seems a little thinner than I was expected, but overall I would recommend them.
