If I were in this situation, I would board at the end of the A1-30 line, splitting the difference between the A-list BP and family pre-boarding. With nobody directly behind me, the transaction would be just between me and the Ops Agent, and I might explain that I was meeting their policy halfway.
I don't know about this, nsx.... I agree that it would be advantageous not having anyone behind you so you can deal with the GA one-on-one, but it may take more time and explanation to talk about the "meeting the policy half way" idea.
I'd say your explanation will be just as well understood if the "A" pax boarded in their original position with child in tow.
As an A-lister, I've seen increasing hostilities at people trying to cut in line. People are getting territorial over the A1-30 slots.
But I doubt that anyone would hassle a parent bringing one 4-yr old into a higher boarding. I know I wouldn't. I think tensions build when people try and bring another adult or one parent with the rest of his family trying to board early rather than using family boarding between A and B groups.