The number words can be difficult for young children, especially the "teen numbers." A Teen number is defined as "ten and some more." So what is eleven and twelve? In some countries the words actually match the quantity. China has students saying er for two, shi for ten and shi er for twelve and er shi er for twenty-two. Compare that with our language and no wonder the United States is a year behind in math at a very early age. Have your children or students count daily.
The words of math are important and children need to have what is called "a stable order." That means that the child can say the numbers in the correct sequence and does it each time. Listen for numbers that are left out and stop and help your child/student so that the error pattern does not continue.