These days more patterns are doing away with age groups and just listing chest sizes in their knitting patterns, and making it even harder by just putting centimetres and not the corresponding inches. Yes I know NZ went metric over 40 years ago but inches were easier for some things and are still used in knitting and quilting...

So here's any easy way to translate those pesky patterns... the "to fit" measurements are the child's chest measurement over a singlet at most, and any "actual garment" measurement is the finished size - the difference between these is known as "ease" - but more about that later.

BABYS sizing

Age - Months 0    0-3   6-9   12-18   24   months
To Fit - cm     35   40    45       50     55   centimetres
To Fit - inch   14   16     18      20     22   inches

The 14" will often fit only small to medium babies and not for long so a 16" will last longer, the other ages are approximate because babies vary so much so it's best to get an actual measurement before you start knitting.

CHILDS sizing

 
Age - Years     2    4      6     8     10    12    years
To Fit - cm      55   60   65   70    75    80    centimetres
To Fit - inch    22   24   26   28    30    32    inches

An easy way to remember for average children's sizing is that 22inch is 2 years old. 24 inches is 4 years old etc
For children the chest measurement is 20" bigger than the approximate age - this makes it easy to remember! There is no such thing as an 'average' child but if it's a surprise and you can't measure first you can get a good idea from clothing size the child is already wearing or comparing to a friend's child and asking what size they are.