I've bought these packs twice. Please know that they do run small, as other reviewers have said, and they are a little thin, which I would assume for an undershirt. This is a fair price for a typical undershirt pack. If you're looking for more of a day regular t-shirt, I suggest buying a regular t-shirt.

We've used them both for fun little art projects with the kids or for just pajama tops to go with lounge/pj pants and they work just fine for those things. I get a size Medium for my 6 year old. Here are my recommendations for sizing:

S: 3/4
M: 5/6
L: 7/8

I would be guessing for the XL, but I imagine it would likely be follow the pattern and be a 9/10.