Adding opening in circular yoke

I’m knitting my second child’s fairisle jumper, the first one looked great but I had to steek an opening around the neck after it was finished as it wouldn’t stretch over the child’s head. This time I’m going to plan for the big heads in our family by fashioning a small opening …but what’s the best way?

Welcome to KH!
You could leave one shoulder open and put a buttonhole band on it. This is used on baby sweaters but you see it on children’s sweaters too. Here are a couple of examples:

I’m not sure what kind of small opening you’re thinking ab

I just wanted to know if I could make a buttoned opening by ceasing to knit in a circular way and turn at the gap I want to create, adding a small button band on each end.

Yes, you can do that. Some patterns add buttonhole and button bands for the opening and some create an overlap at the opening.