A poor explanation of Karma
Karma's not as simple as this cartoon suggests. Buddhism teaches that things happen for lots of reasons: weather, heredity, nature and karma. Karma's different from the others, though, because we have some control over our decisions and actions. Unlike weather, genes, and the natural order of things, karma is about choice. We can choose to do good, or bad, or nothing.
We can't change nature or our genes, but acting with compassion and attention will make a difference.
Here's a way to introduce a toddler to karma without ever mentioning the word.
With your child, pick a or bowl or pot. Together, go outside, find some dirt and fill the vessel. Do it with great attention, describing and experiencing together how the dirt looks and feels -- its texture and weight and color. Put the pot in a special place and let it sit for a day or more, commenting often on the wonderful things that will happen inside the pot later. Go out with your child and buy some radish seeds. Read the instructions on the packet out loud and explain them to your child. Together, plant a few of the seeds very carefully, add the right amount of water, and put the pot on a sunny ledge. Because radishes germinate very quickly, you won't have long to wait.
Explain to your child about all the things that have to come together to made the seed grow: soil, water, and sun, and someone to plant and care for the seed.
As the radish grows and matures, you can refer to it again and again as you teach the child about cause and effect, pointing out that good deeds (planting, watering, putting the pot in the sun) help the seed grow.
Maybe you'll get a nice edible red radish after a few weeks. The radish will offer new opportunities. What is the best thing do with it? Eat it with great mindfulness?
Maybe you won't get a radish. How will you explain that in terms of cause and effect? Maybe you did everything for the radish, but the sun was too dim or the water wasn't quite right, or the soil was not ready.
Either way, your child can learn about cause and effect, and you will have a touchstone for future teaching. You can provide a context for many of your child's experiences when you start an explanation with "Remember the radish?"