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.
No problem at all. Single speed bikes with coaster brakes are very common in the Netherlands and a lot of them have child seats in some form or another. I use one myself and while one does feel the extra weight, especially on the odd incline, it's still very much ridable.
The fixed-gear generation having aged into parenthood, one sees such bikes commonly enough in Brooklyn.
I just used a singlespeed for a short loaded tour because my touring bike is broken. It was fine.