Measure your kids always before buying a new garment. It's surprising how fast they grow! Choose the size according to the result you got. Note that this chart shows an average age. Choose the size according to your kids height.

Height ~ age Clothes size
50-56 cm 19.7-22 in 0-2 m 50/56
57-68 cm 22.4-26.8 in 2-6 m 62/68
69-80 cm 27.2-31.5 in 6 m-1 y 74/80
81-92 cm 31.9-36.2 in 1-2 y 86/92
93-104 cm 36.6-40.9 in 2-4 y 98/104
105-116 cm 41.3-45.7 in 4-6 y 110/116
117-128 cm 46-50.4 in 6-8 y 122/128
129-140 cm 50.8-55.1 in 8-10 y 134/140
141-152 cm 55.5-59.8 in 10-12 y 146/152
153-164 cm 60.2-64.6 in 12-14 y 158/164