Kids love a good joke. Unlike adults who may have developed over time, a mental shield if you will, that doesn't find the same jokes funny. For kids, the best jokes are probably the most simplest of jokes or the silliest of jokes. There are some that even enjoy gross jokes But telling a kid a funny joke isn't all in the joke itself, like doing a presentation, it's all about the delivery such as your tone of voice and body language.
Go for the most simple of jokes, something that doesn't require a complex knowledge of a particular area. For example, you need to understand that kids don't have a complicated vocabulary, therefore jokes involving satire, complicated irony or intelligent puns will go wasted. Therefore, to tell a good joke for a kid, one must go for jokes which involve everyday objects and people. Here's a good example -
What superhero uses public transportation?
Bus Lightyear