What do these gifts tell us about Mr. and Mrs. Campbell?

They're concerned about what will happen to David, and they want him to remember where he came from. The gifts are symbols..... they're life lessons and morals to remember. 

And I sat down on the boulder the good man had just left, and opened the parcel to see the nature of my gifts. That which he had called cubical, I had never had much doubt of; sure enough it was a little Bible, to carry in a plaid-neuk. That which he had called round, I found to be a shilling piece; and the third, which was to help me so wonderfully both in health and sickness all the days of my life, was a little piece of coarse yellow paper, written upon thus in red ink:

"TO MAKE LILLY OF THE VALLEY WATER.—Take the flowers of lilly of the valley and distil them in sack, and drink a spooneful or two as there is occasion. It restores speech to those that have the dumb palsey. It is good against the Gout; it comforts the heart and strengthens the memory; and the flowers, put into a Glasse, close stopt, and set into ane hill of ants for a month, then take it out, and you will find a liquor which comes from the flowers, which keep in a vial; it is good, ill or well, and whether man or woman."

And then, in the minister's own hand, was added:

"Likewise for sprains, rub it in; and for the cholic, a great spooneful in the hour."