Amazon EssentialsGirls and Toddlers' Short-Sleeve T-Shirts, Multipacks

What size do I order for age 7/8. Sizing chart is not explaning s m l etc

Size chart is confusing. which one should i order for my 9 year old girl? xs or s or m ?

What size would I order for an 8-9 year old? M, L etc is difficult to judge

My daughter is a petit 7 year old would 6-7 or 8 years be best? How generous is the sizing ?