An easy way to help is to count alternatively, starting with your child saying, “one” and you saying, “two”. In this way, you will supply the decades and your child will repeat each one five times (as she says, for example, “51”, “53”, “55”, “57”, “59”, between your counts of the even numbers)
After practicing this way several times, you can start the count so that your child says the even numbers. That way, you will know whether she has memorized the correct sequence of decades. Even when she has, she will sometimes make mistakes.