This family is not part of my Aird family.However, you can find this family on the Scotlandspeople website: William and Mary married 14.11.1806 in Kirkmichael.There were 3 children older than James-David born/bapt 30.8.1807, Mary born/bapt. 9.8.1809 and William born/bapt. 2.7.1811.James was born/bapt. 8.10.1815 and I also found John born/bapt. 20.11.1817.(this corresponds with his being 17 years old when he arrived in Canada).I did not find any other children born to William and Mary in Kirkmichael.If there were others, as you say, they may have been born elsewhere.
As James was not actually the eldest of the children, you may find that the 3 older children did not emigrate. They may have remained in Scotland and married there.You could check this on the Scotlandspeople website. Good luck