I worked it out, I'll post what I did for the record since it worked beautifully. Now if only I had a baby handy to try it on..

When I got to the gathering row, I did one single row of gathers. I didn't do three at one time, but { insert hook, yo; insert hook, yo, pull through all loops } down the whole row. Then I sc a row on top of that. Then I did {insert hook, yo; insert hook, yo; insert hook, yo} on each end of the next row, with sc between. Maybe sounds a bit confusing, anyway it worked for me and I'd recommend the pattern highly. Looks really good, and is nice and thick.