To make cakes for a diabetic kid, you can make the buttercream with cooked milk/flour base. I make it with cornstarch instead of flour (same amount) because it's easier to get smooth. You then add Splenda in place of sugar, and ONLY enough to make it taste good...the milk pudding base really makes it hold together without the sugar playing any significant part.
I have done the online search for the Pillbury sugar-free frostings and they use a similar recipe except with various other sugar substitutes and shortening in place of butter. The Pillsbury cake mixes are similarly put together, and you just can't buy most of the listed sugar substitutes in small quanitities--so use the cake mixes.