Here are a few things I've learned from our kitchen time:
1. Make sure your young helpers can comfortably and safely reach.
We started just using a stool or a chair to be able to reach the counter. That quickly became too precarious. We love our Learning Tower. It is adjustable and sturdy. There is room for two or even three kids on it and I can slide it around the kitchen. When she isn't cooking, Addy keeps it by the window. Then she can keep her eye on Suka and the cats or play underneath it. Kylan can stand on it to help wash dishes. It has been totally worth the expense. You can buy them new or watch for them on Craigslist. I saw a couple on there recently for about $60 less than I spent.
2. Forget about the mess. I just don't worry if she sloshes stuff out of the bowl or drops it on the floor. I try to give her advice or demonstrate how to stir or pour more carefully but she's still learning. I keep kitchen towels in a drawer that she can reach so when we're done cooking she can grab one and help clean up. I also use 1:4 vinegar to water plus a couple of drops of tea tree oil to wipe the counters with. That way not only is our cleaning solution better for the environment but if one of the kids comes in contact with it it isn't a big deal.
3. Relax about precision. Although we talk about measuring a lot and usually cook from a recipe I've learned that if not all the flour or what have you makes it into the final product it isn't the end of the world. The best coffee cake I ever made was missing an unknown quantity of the dry ingredients thanks to some enthusiastic stirring and it still rose and tasted delicious. That recipe has never come out as tasty.
4. It probably doesn't need mentioning but I'll say it anyways: Go with the flow. As all parents know, just because you think it is time to do something doesn't mean your toddler agrees. If Addy starts out strong but loses interest half way through a creation I just let her go do her thing or help her get started on something else and finish up on my own. Usually she'll still hang out in the kitchen with me and I'll just continue to narrate to her what I'm doing. That way she's still picking up on the vocabulary and doesn't feel that I'm ignoring her.
Here's a link to the coffee cake. Sour Cream Coffee Cake I suggest adding finely chopped peeled apples to the middle portion of streusel.