So to help illustrate this point, we made a 3-D model cell! This was an extremely easy project that was perfect for a preschooler attention span. I followed the guidelines here, but I know I've seen this activity written up elsewhere around the web as well.
What you will need:
Light corn or karo syrup
Two sandwich baggies
Assorted foodstuffs and toys for organelles
Picture of a cell for reference--I found the one below to be the right level of detail for our purposes:
What to do:
First put one baggie inside the other (there's your bilayer membrane!), and then fill with syrup (cytoplasm). Now, go crazy adding in your organelles!
I spent a fair bit of time organizing my ingredients, and trying to assign an object to each cellular component. I attempted to explain what each thing represented, and gave a very brief description of what it's role in the cell would be. Here's what we used:
And here's our finished product:
So, in the interest of full disclosure, my son was much more interested in eating leftover organelles than learning about the mysterious workings of the cell. I don't think he really absorbed anything other than "mitochondria make energy for the cell" and the term "golgi " (because come on, who doesn't find that funny?). However, we had a good time working on this together, and he loved squishing things about when it was done. I think this would be a fun activity to try again in a few years, when he will perhaps understand a bit more about basic biology.