Repetition can be good when it is used for an intended purpose, such as constantly repeating a message to a child. One example is repeating good and bad practices for when and where to cross a road and when and where not to. Because of the repetition, your child is more likely to remember the good and bad practices when they are confronted with the good or bad options.
Repetition is particularly good when it carries a meaningful message. The classic and most famous example is the "I have a dream" speech made by Martin Luther King in 1963. The phrase was well chosen because it carried more than merely the words used -- it carried the implicit and underlying message of the whole speech. So the phrase is remembered and recited and the underlying points contained in the rest of the speech are brought to mind just from the four words that are used.
Charting a Course, from the Chief of Naval Education and Training, states that the use of repetition "helps learners retain facts, labels, lists, rules, and procedures in their long-term memory." The usefulness is in retaining information such as given facts, rules and lists. Research quoted by Charting a Course suggests that whereas 70 percent of information was retained after nine weeks by learners who had verbally repeated information, only 14 percent was retained by learners who had not recited it.
When your child is young, useful practices can be learned by repetition. Older children and adults can also learn usefully from key points made by verbal or written presentation and repetition. However, you should not confuse this with constant repetition of wider learning materials, which is called rote learning. According to K12 Academics, an educational website, rote learning is a technique which enables the learner to recall the material without any understanding of the issues or "complexities" of the subject. This is not good repetition.