If a child knows how to count, then she can be taught to read a ruler. Start by drawing number lines together on big pieces of paper. Let the child participate in marking spots on the number line, and writing the numbers above those marks, in sequential order. This gives the child practice in both counting and writing numbers correctly.
Talk to the child about how people may have originally measured things with their body parts--that once the length of the thumb and a person's foot was a common unit of measurement. Have some fun with this. How many "feet" long is the room you are in? Let the child walk heel-to-toe across the room, and answer the question on the length of the room. Measure other rooms with feet, and smaller objects with thumbs.
Bring out a ruler or a tape measure. Point out that it is just like a number line, but the spacing between each number is equal. Instead of thumbs, we now use inches. And every 12 inches is a foot. Measure the same things all over again, but this time compare thumbs to inches, and "feet" to feet. So, if a book was 10 thumbs long, how many inches? If the same book is 8 inches long, which is longer, an inch or your thumb?
Instruct the child to choose 5 things to measure, and let her measure them. This will cement the lesson in her mind and give her practice.
Make a chart comparing the things you measured. For example, if a room was 30 child's feet long, how many 12-inch feet long was it? Make a chart showing each measured item in body measurements and ruler measurements. Display the chart in a prominent place.