I might suggest putting an old oversized shirt on your child for decorating. Some icings have dyes that could ruin clothes. I guess cookie dough is a bit like playdough, so I might keep the whole kitchen table open for cookie playing and lots of flour on hand to keep from getting so sticky, lots of wet wash clothes. You might expect to need to mop the floor afterwards...lol. You could do little stations of sprinkles, icing, cookie cutting, flour, etc.
With my boys they like to use their imaginations instead of just using things for what they are made for. You might look around at their toys and see if there's anything they can make imprints with such as plastic blocks, a toy hammer set, a matchbox car, straws, etc. Some of these things are easy to clean and disinfect, or just shove in the dishwasher. They can use different cooking and baking utensils to make neat designs in the dough for cooking and then decorate afterwards.