Every child I've had in The Sims 3 has eventually had a wish first to buy a bake sale table, and next to earn money selling baked goods.  The first few times I attempted this, I had terrible luck!

My luck changed with a combination of finally getting the knack of it, and I think some bugs getting fixed in a patch.  I used to set my kid out at the table, only to stand there for hours on end without a single customer. 

Suddenly now when I set out the table and staff it, adult Sims come flocking.  I'm pretty sure this is something they fixed in the game, and not due to a change I made in my gameplay.

That being said, there is a definite workflow to the bake sale that you have to follow in order to have success.  First, your child needs to bake some things!  They can do this with the toy stove (found under Kids -> Toys in Buy Mode).  Their first few attempts may turn out poorly, just like an adult Sim cooking for the first time with a real stove.  

The more your child bakes, the better they get at it.  Soon they can progress from simple vanilla or blueberry muffins all the way up to brownies, pie, and chocolate cake.  (No, I don't know what a "vanilla muffin" is, either.  It sounds pretty bland!)

Have your child bake a few plates of whatever strikes your fancy.  Drag these things into your child's inventory, and from there they will be able to stock the bake sale table.

Next you need to figure out what price to set.  Have your child read the newspaper to check the going rate for baked items.  

Buy the Bakers Half Dozen Table, and put it in your child's inventory along with their fresh baked goods.  You're ready to go!  You can set up the table either in front of your house, or on a community lot.  I recommend setting it up at the park.  Not only is it a pleasant setting, but it has plenty of room to keep customers from crowding and blocking each other.

Click on the table in inventory and choose "Place."  Set the table, then click on the table and choose "Stock Goods" to add the baked goods to the table.  Then click on the table and choose "Set Prices."  You can fiddle with the prices a little to play with supply and demand.  Frankly I usually set the prices pretty low.  It's not like the kid is supporting a family or anything.

Finally, click on the table and select "Tend Table."  Note that you have to specifically tell your child to work at the table!    This was my downfall a few times in the past.  If you don't choose "Tend Table," adult Sims will show up, and your child will stand nearby, and eventually everyone gets angry and leave.

Once you have run out of either baked goods or patience, you can click on the table and choose "Collect Money."  Then pack everything up and head home!