I set out to purchase some basic white t-shirts for my son to wear under his pajamas, as his room is the coldest one in the house. It took me a while, because few people actually make 3-4T plain white t-shirts. Finally I found some made by Fruit of the Loom so I bought an entire pack.
I purchased a size 4T and my son is wearing a good 3T (he will be three soon). I purchased a size larger because I like the shirts to fit a little bigger so he can grow into them. I put the t-shirt on him and it was tiny. I always wash my kids' clothes before they wear them, and I did that with the T-shirts so there is a chance that it would have fit him better had I put him in it before washing it. They are all cotton, so chances are it shrunk a little bit but the 4T shirt now fits more like a size 2T. Needless to say I am very disappointed.
The other aspects of these t-shirts are fine. The material is soft and the price is reasonable. If you do choose to buy these t-shirts for your pre-schooler just buy at least two sizes larger than you need.