It's tough to cut the sweetness of ANY icing, as all are mostly sugar. All I can suggest is adding some lemon juice (or cream of tartar; something acidic) to cut the sweetness a bit. Glycerin, glucose, and corn syrup can all be added to soften the bite of royal icing, but glycerin is most effective. Please see the "Toolbox Talk" blog post about "Corn Syrup" (under the Blog area) for all of these details. This article compared all three as softening agents. (Note, though, that the tradeoff of these additives is that the icing may never set all the way through, and thus it may become more difficult to stamp or stencil on the cookies. These additives can also increase the spread of the icing.) Please also search our "forums" (using the magnifying glass icon in the top right of the site). I am pretty sure this topic has been addressed in at least one other thread.