Obviously, there is no answer that is right for everyone - a lot comes down to the situation, parenting style and so on. Also, you did not mention the child's age, so I'm assuming they are about to start elementary school, so about 5-7 years old. With that disclaimer, my advice would be:
Let them walk, with or without you.
First of all, walking is beneficial to everyone, particular to children - see e.g. Physical activity guidelines for children and young people by the NHS.
In particular, for small children, in adddition to the general benefits from exercise, walking to school also:
- allows them to get to know their environment
- allows them to be more independent (particularly if they walk alone, but even with you, you can let them lead the way)
- allows them to socialize with others
So, if at all possible, let your child walk to school. You can walk them during the first weeks, and if you are satisfied they know the way and the challenges (traffic, other dangerous spots, etc.), they can walk alone.
As an anecdote - my daughter always walked to school, starting at 6 years of age. I walked with her the first weeks, then she went alone. In her case it was less than a kilometer, however.
To address your concerns:
If their parents walk them there that is safer (especially at younger ages with traffic accidents)
Yes, accidents are a problem - but that can be mitigated by walking the child at first, and making sure they know how to be safe and are aware of dangerous spots (such as difficult intersections). Also, they can be injured while in the car, too.
Driving can save some minutes of (their) sleep especially in the morning
True, but I believe the benefits outweigh this disadvantage - plus getting exercise and outdoor time in the morning may be more beneficial than an extra 15 minutes of sleep. Or just send them to bed earlier :-):