Nigella does a good one. Butter biscuits. I made them on the weekend. Very easy and the dough freezes so I've got some in the freezer for later.
175g soft butter
200g caster sugar
2 large eggs
1 teaspoon vanilla extract
400g plain flour
1 teaspoon baking powder
1 teaspoon salt
300g icing sugar
food colouring
Preheat oven to 180 Celsius/gas mark 4. Cream butter and sugar together until pale and moving towards mousiness, then beat in the eggs and vanilla. In another bowl, combine the flour, baking powder and salt. Add the dry ingredients to the butter and eggs, and mix gently but surely. If you think the finished mixture is too sticky to be rolled out, ad more flour, but do so sparingly as too much will make the dough tough. Halve the dough, form into fat discs, wrap each half in cling film and rest in the fridge for at least 1 hour. Sprinkle suitable surface with flour, place a disc of dough on it (not taking out the other half until you've finished with the first) and sprinkle a little more flour on top of that. Then roll it out to a thickness of about 0.5cm. Cut into shapes, dipping the cutter into flour as you go, and place the biscuits a little apart on the baking sheets.
Bake for 8-12 minutes, by which time they will be lightly golden around the edges. Cool on a rack and continue with the rest of the dough. When they are all fully cooled, you can get on with the icing. Put a couple of tablespoons of just not boiling water into a large bowl, add the sieved icing sugar and mix together, adding more water as you need to form a thick paste. Colour as desired: let the artistic spirit within you speak, remembering with gratitude that children have very bad taste. Makes 50-60.
For your info I doubled the mixture which filled my big mixing bowl. I didn't create more dishes for myself by using two bowls but just put all the dry ingredients into the wet stuff and stirred it around. I found the mixture very moist and ended up adding quite a bit more flour. I didn't roll mine out as thinly so they have a little bit more substance. I used big cutters in a variety of shapes but if you don't have any then use a glass and hunt through your Childs toy box for odd shaped things (including building blocks) that they will want to use. It will personalise it a lot more for them if they are using their toys to cut out the shapes.
I didn't have food colouring in the house so sprinkled chocolate flakes on some and hundreds and thousands on the others.
These are very easy to make and your child will love to help you. I remember when I was young and used to cook (with mum standing by) she always made me do the dishes so I learnt the value early on of keeping a clean place as I cook, using less dishes and tidying up after myself. I never minded as I thought this was just what you had to do. I did the same thing with my niece when I was last home who helped me bake some sponges. She didn't want to do the dishes but I told her that she had to help as that was all a part of cooking and she had helped cook.
I also have a really good chocolate truffle recipe that you have to get your hands into and get all dirty for that the kids would love. I'll hunt around as it is really simple to make and they can crush the biscuits first with a rolling pin which they will also enjoy.