IMHO I would start with what is called a "comfort bike". I would not worry about gears right now. It only makes it confusing and could lead to frustration. Let me put it this way. Current model year bikes are 27 different gear combinations you can use. As another poster said here make sure your frame is compatible with a child carrier or go with a trailer. I would reccomend a trailer. They can hold more weight, and have more protection from the elements. As far as model of bike, well go to a bike shop, not a department store and tell them what YOU need. Do not accept what THEY want you to have. Ride everything you can, and decide on which one is more comfortable. Mountain bikes get you leaned over alot. Road bikes you are leaned over even more. Hybrid's and cruiser's get you more upright and can be more comfortable to ride when you first start off. Remember the frame is the most important part of a bike. If you are going to pull your child get a sturdy frame. I can tell you to stay away from straight gauge hi tensile steel. Go with a Cro-Moly frame it is sturdier lighter and will last longer. If and when you decide to do Trianthalon's you are going to require much moe bike. That is something to look at down the road. Some manufacturer's I can reccomend for cruiser's are Schwinn, and GT. Here are some links to check out.
http://www.gtcomfortbikes.com/catalog/index.html
http://store.schwinn.com/cgi-bin/nco...CGRY_NUM=49081