Maybe you should try the fruit and orchard forum. I don't know of any tree that is that fast to go to fruit, it takes at least a few years. Maybe some kind of berry like a strawberry? What about peppers or cherry tomatoes or beans? See if they have some better ideas over there. Also there are a few other edible type forums here you might scan through.
Maybe you could call a company like Pinetree Seeds or Johnny's and see if they can recommend something. Also, some of these seed companies have days to harvest in their catalog. Radishes and lettuce grow very fast but I'm not sure how much kids will like a radish. At least they are colorful. There are colorful lettuces too.
I grew crabapples with my kids once. We germinated the seeds in the refrigerator in a moist towel placed in a ziplock. It took 5 months just for the roots to show. After that, they only grow a few inches a year until they take.

Here is a link that might be useful: fruit and orchard forum

avocados?