I wonder how much extra you could charge for having a vehicle in the uber fleet with a variety of child seats, or similarly various handicapped accessories.

Answer is pretty easy to find out:

http://blog.uber.com/uberfamily

Also:

https://shuddle.us/

Given that cabs last a long time (i.e. can amortize the cost) it shouldn't be too difficult to make an adjustable child seat eh?