Choosing the wrong bike can put a kid off cycling for a long time. It's heartbreaking to see a kid pushing a bike up a slight slope because it doesn't fit, the gears don't work properly, and it weighs a ton because they (or their parents) chose a cheap imitation of a downhill mountain bike because it looks cool and has loads of "features".
Make sure it is the right size and sell it on or keep it for a younger sibling when they outgrow it. If it is the wrong size then it is harder to control and they are more likely to crash. Make sure they can reach the ground when the saddle is set properly for them, and they can still steer and control the bike.
Weight makes a big difference to the fun and manoeuvrability of a child's bike. A kilo saved from the bike of a six-year-old weighing 30kg is like 2.5kg saved from an adult's.
Suitably sized cranks will make a big difference to a kids ability to pedal well. If the cranks are too long then the bike will need to be higher off the ground to prevent pedal strikes, which also affects handling.
Depending on the size of your 6 year old they will either require 16" or 20" wheels. With 20" wheels you can have gears - a 3 speed hub is easy to use and hard to break, but more expensive than derailleurs. No harm in getting singlespeed for a nice light and easy to maintain bike.
If you let the kid choose the bike, invariably they will choose one with suspension. This tends to add weight and cost money, without providing much (or any) additional comfort or benefit due to poor construction.
Lastly, check that the reach of the brake levers are adjustable for smaller hands, and the brakes still function when adjusted so that they are closer to the handlebars. A back-pedal coaster brake is a good solution if your child is a bit small to operate a rear v brake (which are harder to pull).