Boys' Formal Black Suit with Shirt and Vest

What do the sizes mean? I see the a 12 , which is what my son is ( a big boy 12, husky fella) but is that 12 in toddlers or ...?

What would be good size for a 13 year old girl to wear this?

Is the pants skinny fitted?

10 year old slim. What size to get? Shoulder length 13 in, arm length 19.5 in, jacket length 19-20in, inseam 21in and pants - 32in.. waist 22-23in