Originally Posted by jsdavis

If the hub on the front wheel has one of the following things on it then you can put a rotor for disc brakes on easily. If the wheel doesn't have 6-bolt holes or Centerlock splines, then you'll need a new wheel. The caliper will go on protrusions on the fork on the non-drive side (the side that doesn't have the gears and chains). The same will apply for the rear.

Note the 6 bolt holes






Note the splines on this Centerlock type hub


These are the protrusions required on the fork or frame for disc brakes:



You could always try riding your bike and trailer around with a bag of rice or flour or something like that to see how it goes. That way you won't have to worry about endangering your child first.

As far as upgrading brakes, even if it's possible, you're looking at $50-60 for the caliper and rotor not including labor. This is for a Avid BB7 disc brake. If you need a new wheel then that will be maybe $60-100 so it can get expensive. Replacing the hub is probably just as much, maybe even more, because it requires rebuilding the wheel which take a bit of work. The spokes have to be tensioned so that the wheel doesn't wobble side to side and such as it rotates. Wheel building is one of the more more difficult tasks when it comes to bicycles.

You can always wear bike shorts under other shorts or pants if you don't want to ride around in tights. Mountain bike shorts tend to have a tight liner with or without padding and come with baggie shorts outside sometimes as a two piece arrangement. Oh and you don't wear underwear under bike shorts.

In San Francisco people ride around in just about anything because bicycling is used for transportation despite the hills. They wear bike specific clothes, suits, jeans and t-shirt, and even nude sometimes.