Think back to the last time your doorbell rang and your child ran to the door to open it.
This can be dangerous if it's a stranger but your child is too short to look through the peephole.
Most peephole heights are made for adults but a child's curiosity and energy might beat you to the door.
A peephole set at about 36 inches from the floor would allow them to use the peep hole without having to grab a stool or chair.

1. You need to find the thickness of your door.
   Simply measure this with a tape measure or ruler.
   Most peepholes allow a range for depth so exactness isn't required but you'll need to come close.
   
2. You need to determine what material your door is made out of.
   Most front doors are solid wood metal.
   Make sure you have the right type of drill bit of your door.
   
3. Using a tape measure, measure 36 inches from the floor and then half way from the edge of the door.
   Mark it with a pencil as a pen doesn't write well on metal well and can bleed on solid wood.
   
4. Pick up a peephole from your local hardware store with the right depth.
   If you don't have the correct drill bit, make sure you get one the right size for the peephole.
   
5. You want to drill a small hole through the door on your mark.
   This is called a pilot hole and makes it easier for the larger drill bit to be accurate.
   Try to make it as level as you can.
   
6. Once your pilot hole is drilled, use the larger drill bit for the peephole size.
   Drill all the way through the door as level as you can.
   Brush off any debris from the outside and inside of the door.
   
7. Put in the peephole.
   Most peepholes come in two pieces that twist apart.
   Feed the lenses part from the outside and then twist the eye scope part into it from the inside.
   

And that is it.
You now have a safe way for your child to view who is at the door without opening the door to satisfy their curiosity.