Did you look at the microfilm of the actual parish records for Laggan? Since that is where the marriage took place, it seems the most likely place to start. There is an Alexander Davidson, married to Isabel Cattanach, who had children baptized in Laggan in the 1780s. while there doesn't seem to be a son Alexander, it's rare that an entire family's birth records made it into the parish records--some children inevitably aren't there, or the records were badly kept or lost.
If the family is from Laggan, you will probably learn some useful details by reading Letters from the Mountains, written by Anne Grant who herself was from Laggan. It covers the right time period and is available on line at Google Books and doubtless from other sources.
