I have researched most of the MacKinnon families around Lake Ainslie and I can't figure out if/where Isabella fits in. It doesn't help that I know very little about her or Malcolm. The only child I know off is Jessie born July 28, 1827 at North Lake Ainslie.
On coming from Pictou Malcolm settled at North Lake Ainslie. He later sold his land to Charles MacLean, son of Allan. Apparently they went to Upper Canada (Ontario) but I was never able to successfully trace them. Would you be able to provide links to the '51, '61, or later census records?
You can contact me directly at aodhqu at gmail.com