I finally completed my totally cheap kid carrier yesterday. The bike is an old steel Schwinn Frontier mountain bike that I found at a garage sale last summer. I spent some of last fall, and a bit of time over the winter, cleaning and fixing it up. I had to completely tear it down and put it back together again. The rear derailleur was too far gone to fix, and I splurged for a set of 2" Serfas Drifter City tires to make my 30-mile round trip commute easier.
I grabbed the 6-speed Burley Piccolo off of Craigslist a couple days ago. It was in excellent condition, with only the little flag missing. I'll pull the flag off of my old bike trailer later.
The Schwinn will be used for commuting to work on the weekdays, and taking my daughter out grocery shopping and to the local playgrounds on nights and weekends.
Schwinn Frontier: $25.00
New rear derailleur: $18.00
New tires: $50.00
Burley Piccolo: $50.00
Lots of elbow grease: FREE
Total: A complete kid carrying and work commuting setup for less than $150.00.