Synchronization

Ask kids to draw 20 circles together. If working with kinder gardeners don't have more than groups of 3 people initially.

The first solution will most likely be incorrect - at least one kid will draw 20 circles, and the others will draw as many as they can.

Point them to the incorrect solution, also, time them - to appreciate how fast it is to draw circles without synchronization

After several tries kids should start synchronizing. Initially it will be event driven synchronization. After about 5 circles one kid will take control and stop everybody to count the circles.

Then he will monitor closely the process, most likely re-counting all circles from the beginning after a couple of new circles. 

I experimented with 2-3 kids. While it took them 30 seconds to draw almost 60 circles, with synchronization it took them almost 3 minutes to draw 20 circles.

If there are many kids, consider splitting them in teams and competing against each other.

Look into different ways of synchronization:

Event driven. "How many do you have?" "Stop, let me finish the rest"

Synchronized counter.  Every kid writes the next number in a shared sequence after drawing one circle.

Compare to drawing circles writing numbers is way too slow. So they may want to draw something more complicated, like houses.

If there is a large group of kids, nominate somebody to be a dedicated counter - whoever draws a house raises a hand and the "counter" counts the number of hands. (The boundary may be a problem when 1 house is left and multiple kids are drawing it)

Split the work. If the kids draw equally fast, this is easy, what if somebody draws twice as fast as the other kids? How do they utilize "all the CPU"?