15" diameter (round) would be my suggestion, but you could certainly go a bit smaller. While it seems too small for most adults, on a child's kayak it will allow room to grow without becoming too small as quickly. You'll have a bit of the deck beams showing but that's not unusual.

See the child's kayak in the link below - from the drawing, its coaming appears to be about 13" diameter.

: 15 inch betwene gunwales and 13 in frome masicks to next rear beam. what
: should my cockpit diamiter be?

: Thanks,
: Sage