Few people enjoy grammar. Teaching Chinese grammar can range from having your child memorizing all the different Chinese classifiers (量詞) (there are so many that even native children have trouble!) to taking a class or going through a Chinese grammar textbook.
You may choose to do this yourself, outsource to a tutor, or find Chinese grammar videos online. Start with the grammar rules that provide the biggest return for the amount of time you invest.
For example, though it may be tedious, learning the most commonly used classifiers will make a huge difference and immediately elevate your children’s Chinese to a more native level. If only because most non-native speakers default to the measure word 個/gè/ for everything and use 雙/shuāng/pair/ instead of 條/tiáo/(classifier for long, thin, narrow objects) for pants.
Another trick (especially for older children who have studied some English grammar) is to teach basic Chinese word order so your kids can sound more native. This may be something you are not consciously aware of and do instinctively, in the same way most native English speakers aren’t ever taught but still naturally know the general order of adjectives.
Take English as an example, the general order of adjectives is marked by this hierarchy: