Caius is 5, 110cm tall and wears size 4-5Y
Koko is 8, 125cm tall and wears size 8-9Y
Archie is 8, 130cm tall and wears size 8-9Y

Garment measurements by size (approximately):
Size (cm) 12-18 18-24 2-3Y 4-5Y 6-7Y 8-9Y
Length 32 36 40.5 46 48.5 53
Front Chest 34.5 37.5 39 41 42 45