***UPDATE*** Got the 1 year extension (well, the "under consideration" stamp) and thought I'd share some of the nuances at Chaeng Watthana.
In addition to
Legitimization documentation,
Condo lease,
Copies of landlord's ID card and house book,
Copies of child's and mother's ID card, house book,
Picture with mother & child in front of condo building ,
Picture with mother & child by front door with unit number visible
Picture with mother & child in the lounge
Picture with child and in front his/her school/kindergarten
Original letter from school acknowledging enrolment
Letter from your bank showing minimum 400K
A hand-drawn map of your house/condo's location . . . . . .
you'll ALSO need a bank statement going back 3 months and a photocopy of the actual bank book. Also, even though I'm told there's no written requirement for the child to actually live with you to get the extension, Chaeng Watthana immigration won't issue it if the kid doesn't.