It’s understandable that you don’t want your little one to have too much sugar if they’ve not had it before.  They actually don’t usually eat a great deal of the cake during a cake smash (sometimes they do but usually not) as it’s more about exploring the textures of it.  However, you can make cake with a sugar free alternative and I have had some clients use fresh cream flavoured with fruit like blueberries and strawberries as an alternative to icing and that’s definitely been very popular.