Henson is 6 and wears am 8-9Y, Harper is 10 and wears size 10-11Y + Akira is 4 and is wearing size 6-7Y for an exaggerated look.
Garment measurements by size (approx):

Size (cm) 1Y 2-3Y 4-5Y 6-7Y 8-9Y 10-11Y 12-13Y
Waist 21 22.5 24 25.5 27 28.5 30
Length 28.5 32.5 37 41 44 48.5 52.5