Found some of these little interconnectable "building blocks" toys - comes as a tube of 72 six pronged interconnected doodads in the dollar section at a popular chain store that is fond of the color red.
I saw how simple they were and decided I'd just measure them up and print a few extras for my kiddo, since I knew they'd likely get lost immediately.
My print time is about 10 minutes per "block", and I've been using Orca Slicer with precise walls and Arachne wall generator at .12mm Fine on an Ender 3 with a standard 0.4mm nozzle and Inland PLA+
Do note that if you print "V3" the center hole interlocks more strongly, making it useful for single pin connections, but the "V2" comes unstuck more easily, which makes it easier to disconnect - it can be very difficult to disconnect two PLA blocks of the "V3" style when they're connected as shown in the third photo, so you may want to stick with "V2" for kids, even if the connections end up being on the loose side sometimes.
Have fun! :)