I think biscuits and cakes can be quite stressful things to make with your children at first, there are so many steps. I found it easier to start with making other things, even something simple like tuna mayonnaise, it's very quick but they feel like they've done something grown up, and because it's so quick they don't lose interest and you don't lose patience. Also breading fish, the same way littlebylittle described making chicken goujons. Fishcakes are nice and messy to make too. Pizza is easy, as you could buy a premade base and just decorate, it doesn't matter if the toppings get eaten a bit.
If you'd rather go sweet I think cakes are easier to stir than biscuits, muffins even more so.
Also, you could try cutting down the amount of steps, for example, weigh out the cake ingredients in advance, then your child can help you tip things in and stir rather than have to be patient while you weigh things. Or make the biscuit dough in advance so all you do with them is roll out and use cutters.
If your LO is a bit greedy, then have a sneaky pot of raisins hidden which you can produce for them to have a munch on or a spoon to lick while you do a step they can't help with.
I always try and make sure I have lots of time spare, that always helps me stay patient!