Kids outgrow their bikes so fast but here is a way to put even the smallest 12" wheel kid bike back into action as a cool-goofy adult bike.
Re-uses/repurposes old items from the dumpster:
small kid bike,
old steel tube,
wood mop handle.
Visit a good primer on Dumpster Dipping
Only had to buy the banana seat and a 5" long 1/4" bolt.
Step 1: Parts Needed
Steel tube to fit in steering tube and support seat nose.
Steel tube that accepts kid bike stem
caster wheel from cart, furniture etc to form swiveling seat nose bracket
1" or 7/8" wood dowel (about 8" long) to connect caster threaded post to steel seat nose support tube.
Crutch to support back end of seat
3/8" bolt to bolt seat nose to caster bracket
1/4" by about 5" long bolt and 5-6 nuts to bolt seat back to crutch
Step 2: Pull stem from kid bike and make a new tall seat support tube instead
Slip it into steerer and determine length needed to get eat to right height for you.
Cut to length remembering to subtract height of caster frame that goes between tube and seat nose bracket.
Shave dowel down to fit snugly in top of stem tube. Pound it in at least 5 inches. Cut off protruding excess dowel. Drill snug whole to let you force thread the caster bolt into the dowel.
Attach stem tube to bike either slipping it directly in fork's steerer tube or using an intermediate tube in the steerer and slipping the stem over the intermediate tube. fit with aluminum can strip shims if needed for snugness.