Why not "grow" a childhood memory?
I like to think everyone has a strong childhood memory of at least one or two specific plants. Do you?
For myself, I have two. One is dianthus because that was the first plant my grandmother taught me to transplant and grow.
The other...well, the other was one my folks didn't know much about. It sounded exotic because the flowers were supposed to open each day at 4:00. How cool is that for a child? Therefore, I bought an entire envelope of Mirabilis jalapa with my allowance to grow along the back fence. What a riotous back fence it became! (And to be truthful, every spring it was bright and boisterous as these plants are difficult to eradicate. However, my parents forgave me each year when the hummingbirds arrived!)
It's a great "first seedling plant." The seeds are large enough for a child to poke into the ground. They germinate quickly (7 to 14 days) so a child doesn't forget about them. They grow bushier if pinched back, giving the child something to do while watering. Soon, brightly colored flowers (white, red, orange, magenta and combinations) cover the three-foot tall plants until frost causes the plant to die back.
At this point, it's easy for a child to harvest the large black seeds, dry them (perhaps share with friends?) and wait until the following spring to start over again.
Ah, if only all childhood memories were so easy to relive...