Are single speed bikes considered good bikes to use with child seats? I wasn't sure if the added weight of a four year old would make a single speed too hard to use. Will be using the bike on relatively flat terrain.
It is unlikely that a four year old will slow you down much on a single speed. Just make sure you've got a well maintained front and rear brake.
On the other hand, if you're thinking about making a purchase, consider the advantages of a bike that has a few gears, maybe seven or ten, etc, if you anticipate tackling a hill every now and then.