1 Have them do things that are age appropriate. A two year old can tear lettuce for a salad, peel garlic, dump pre-measured ingredients into a bowl, transfer chopped up veggies from the cutting board into the pan, etc. A six year old can crack eggs into a separate bowl (inspect for shell fragments before mixing with other ingredients), measure dry ingredients, mix without making a huge mess, quarter potatoes (just make sure you teach him proper knife handling), or roll out the dough.
2 Make it fun. Who says all biscuits have to be round? We have a huge container of cookie cutters that are used for homemade noodles, tortillas, biscuits, and even cookies. Make faces on your pancakes or add coloring to your muffins. That being said, let them be done when they are done 'cause nobody has fun when they're being forced to cook.
3 Don't expect perfection. It's more important for your little helper to feel good about their accomplishment, than for it to be even, round, or even look good in any sense of the word. If you praise them for their efforts, they'll want to do it again and they'll eventually get it right. If you must make some look fabulous, give them their own lump of dough to do with as they please and you take care of the rest.
4 Remember what the purpose of this activity is. The point isn't to make food, it is to make a memory. If you are in a hurry to do it right or within a certain time, it will be a disaster. If you take it slow and exercise a bit of patience-gasp, everyone will have a good experience and want to do it again. The other purpose of cooking with your children: teaching them to eventually do it themselves. Let's face it, they won't always be little ones and will have to start doing it sooner or later- let's make it sooner.
5 When they do it themselves, try not screaming out loud. Along with the creative liberty I give my children while cooking together comes the problem of them trying to do it alone and making a terrible mess. It appears to be a natural consequence of letting them pour and stir by themselves.
(This lovely potion consists of soy milk, ovaltine, hot cocoa mix, pecans, and cheerios)
A few things I let my children do (my oldest is 6 years old):
Roll up the croissants.
Brush butter on the rolls.
Cut green onions with scissors.
Pour pre-measured wet ingredients.
Measure dry ingredients (make sure you are using measuring cups designed for dry ingredients).
Mix- I hold the top of the spoon with the younger one to prevent her from flicking it all over.
Wash produce. (if they tend to splash a lot, place a towel on the floor and on the front of the sink draped over the counter.)
Play fetch- "please get me the Crisco."
Tear the lettuce for salad, stem the spinach, or pull the stems off all the cherry tomatoes.
Roll out the dough, cut the biscuits (or whatever you are making), and place it on the pan.
Crank the handle. They might only last a few minutes at the pasta maker, but it's a break for you and entertaining for them.
Crack the eggs into a separate bowl.
Shred the cheese or carrots with a stand up shredder.
Put the cheese, fruit or veggies into the food processor, blender, or pan.
Clean up- If they can cook, they can clean.