Help with "A boy's own jacket" Zoe Mellor

[miemoo]

Hi,
I joined to get help completing a jacket for my son from "Nursery Knits". I checked the errata post for this book but found nothing for this piece.
I'm not understanding how to proceed with the hood. I would appreciate any help. The pattern reads as follows;
3 sizes; 6-12 (1-2:2-3) I am knitting the 3rd size.
hood
Inc on given rows only.
4th row K23 (26:29), m1, k4, m1 k23 (26:29). 52 (58:64) sts.
8th (10th and 12th) row k23 (26:29), m1, k6, m1. k23 (26:29. 54 (60:66) sts.
12th (16th and 20th row k23 (26:29, m1, k8, m1, k23 (26:29). 56 (62:68) sts.
...and it follows on in this fashion until 68 (74:80 sts).

Since I am knitting the 3rd size what I don't understand is; do I increase on both rows in brackets? In this case, the amount of sts does not correspond. I tried increasing on rows indicated for 3rd size only (e.g. 12th, 20th, 26th etc) but the hood turned out in a strange square shape.
It's so frustrating! Appreciate any help
Mieke