adidasKids Squadra 21 Shorts

What would be the size for a 10 years ols skinny girl?

what size would it be for a 11 years old boy?

Why are the larges 19.99and the mediums are 29.99 ??? makes no sense.

My seven year old son is very thin. he is 50 lbs and 48 inches tall. he wears size 6/7 shorts normally. should i get him a small or xsmall?