There are over 20,000 characters in the written Chinese language. A highly educated person can be expected to know ~8,000 characters. In order to read the newspaper, one needs about 2-3,000 characters.
That is a LOT of unique characters! It is not uncommon for a native Chinese to come across unknown or unfamiliar characters.
This is when radicals can come to the aid.
Like the taxonomy of organisms in science, radicals group characters together under a broad category. For instance, in science, if you say an organism belongs in the Kingdom Animalia, you know it’s not a plant, or a bacteria, or even a fungus. It’s still a wide wide field of possibilities, but you know it’s an animal and have a general sense of what it is.
In terms of Chinese characters, if you see the radical 宀, you know it generally has to do with houses or buildings of some sort. If you see the radical 艹, you know the character generally has to do with plants (but not trees).
List any radical - even the hard and obscure ones, and if you have a general knowledge of radicals, you can have an idea of what the word means or represents - even if you do not actually know the character.
As your child becomes a more advanced reader, the radical will give your child enough of a hint as to what “family” this character belongs to, and given the context of the surrounding characters, as well as the other sound and meaning components of the character, your child has a good chance of “guessing” that character. In fact, this is how more advanced readers eventually pick up characters.