Call me a wet blanket, but I wouldn’t be in any hurry to introduce this idea. For one thing, it’ll be a long while yet before they’re able to exercise good judgment about what is and isn’t a safe or appropriate joke to play. For another, the idea of doing something once a year has no meaning to a little kid. You may be up to here in tricks for weeks.
If you must, do something that’s instantly reversible and not something that requires an inadvisable move such as eating something that looks bad or scary. For instance, try putting their clothes on backwards or serving their milk in a coffee cup. Don’t compel them to overcome some natural instinct or aversion. What’s funny about that?
I would also say emphatically, don’t do something that requires you to tell them any lie. That people think it’s okay to lie casually to children astonishes me.
My mother introduced me to this prank holiday by tipping me to the joke she was playing on her father. They both wore gold-rimmed spectacles. She swapped hers for his where they lay next to his waiting breakfast. Grandfather put them on and couldn’t read the newspaper. She let him puzzle for a moment and then declared “April Fool!” and restored his own glasses. It was funny and harmless and could be set right in an instant.