Las Vegas your 3 yr old could help you make pretty much anything. I know my kids do. I don't limit things to cakes/biscuits. When she was 4 my dd liked helping making roast chicken by rubbing butter all over a chicken and sprinkling on herbs and lemon juice.
However I do scones, coconut pyramids, bread, any biscuits, any basic cakes etc. Choc crispy cakes are dead easy.
Choc crispy cakes are golden syrup and butter/marg melted together (I do it in microwave). I usually do approx equal amounts. eg 2 big sppons of syrup and 2 big spoons of marg. add cocoa powder (2 spoonsful) and enough rice crispies to coat them all (no idea on measurements). Give child teaspoon and apron and let them fill cake cakes.
Any biscuit dough can be done and then the child can roll out and cut with cutters. Or cookie dough where they just need to roll walnut sized balls and squash them. Or muffins where you weigh out dry ingrediants, mix with wet (I let kids do this) and into cases.
I also have a few kids cookery books.
We use Poppy and sam which is quite fun.
Amazon have loads of books look here Beware of American ones!!
I also got a good one from the book people which was sweet. book people
I also use a Dorling Kinderley (I think) kids cook book which has loads of great pictures. I think Big cook/little cook has a book too, but if the recipes are like the one on TV they would be foul.