Sizing for each age range:

2-3 years, to fit chest 56 cm, 22 in (2 balls, 5 buttons)
4-5 years, to fit chest 61 cm, 24 in (2 balls, 5 buttons)
6-7 years, to fit chest 66 cm, 26 in (3 balls, 5 buttons)
8-9 years, to fit chest 71 cm, 28 in (3 balls, 6 buttons)
10-11 years, to fit chest 76 cm, 30 in (4 balls, 6 buttons)
12-13 years, to fit chest 81 cm, 32 in (4 balls, 6 buttons)

Sirdar Supersoft Aran comes in 100g balls, 236 m per ball, so quantities required will be generous in the small size of each pairing.