..PLEASE keep in mind that this info is not concrete, its just a guide. "Toddlers" are from 2 YEARS to 6 YEARS. Its probably safer to go with MEASUREMENTS as opposed to "My child is 5 years old, so they are a 5T." there is a helpful Size Chart inside FAQs under "Sizing Assistant".
However, in MYTEEUSA, there are different brands, different materials and various styles and cuts... so in a related topic, sometimes I can fit in a 10.5 shoe... sometimes I can fit in a size 12? Go figure!? So the description and sizing inside the shirt you selected is probably your safest bet!
In regards to making a GUESSTIMATION, Id always err on the larger side! Children grow like weeds, so theyre most likely going to grow into it...and unfortunately out of it! In the case of larger size for an adult, your never gonna grow out of it, so you could wear something underneath, like long underwear, or another colored Tshirt, OR... it may shrink after a couple washes based on the materials the shirt is composed of?
PLEASE, make sure to do your best, because there are no returns because "I selected the wrong size!" or "I washed and dried it and it shrunk!".