Great Shirt, But Runs Small

This is a great t-shirt, love the design and quality! The only issue is that it seems to run small. Since it's a new product and there was no size guide I used the picture on the website which had a teenager modeling the shirt. The description also mentions that it is a relaxed fit. I ended up ordering a small for my 7 year old nephew and it was much too small, most likely would fit a 4-5 year old. I reordered in a medium because the large was sold out, and the medium is only slightly larger than the small. Bottom line, sizing may be challenging when ordering online unless a sizing guide is added.