A group of children dressed as a witch, a skeleton and a ghost are approaching a house. Since it's Halloween, we can assume that they are trick or treating. This is when children in costumes travel from house to house in order to ask for treats such as candy (or, in some cultures, money) with the phrase "Trick or treat!". The "trick" is a (usually idle) threat to perform mischief on the homeowners or their property if no treat is given to them.
In Mac's cartoon, however, the children are in for a nasty surprise. The homeowner has dressed up as a scary monster (we can tell it's only a costume by the zip on the back). His wife tells him, "Oh for heaven's sake, Donald. They're hoping to scare you!" (I think you have to stress the 'you').
For heaven's sake is an expression used for emphasizing that you are annoyed or impatient with someone. • Oh, for heaven's sake! Anyone would think this was difficult!