🚀 go-pugleaf

How about just using alligator clip leads?  Easy to get, and easy to 
understand.  Also get D cells, LEDs, and a small motor.  The LEDs will
teach your kid about polarity, which the old light bulbs didn't do.

Taping three batteries together and soldering leads across the ends
makes a nice battery pack, though I wouldn't be surprised if nobody makes
batteries with solderable metal contacts anymore.  If that is the case, 
a battery clip is needed.  Three batteries is nice because you can then
power TTL.

Be sure to solder series resistors on the LEDs before giving them to
your kid.

The battery from a Polaroid film pack also powers TTL nicely, using a couple
bent paper clips as contacts.  You need to lower the voltage a little,
for example using a 1N4001 rectifier to subtract a diode drop (0.7 volts).