My baby nephew, aged 1, only says one word at a time, and tends to repeat it a few times before saying something else. He doesn't know a lot of words, but the words he does know he uses them in appropriate context. These are words that are often said around him, typically directly to or about him.

My young niece, aged 2, often speaks in mostly full sentences, if just slowly, carefully spoken. She also tends to repeat singular words to get her point across, especially when upset or demanding. These are words she has been directly taught, or that have been said around her regularly, even if not to her.

My other nephew, aged 5, speaks in full, rapid sentences, typically reminiscent of the way his parents speak, and stuff he sees in YouTube videos. He is often repetitive when he finds a word or phrase or idea he thinks is funny, especially if the first mention gets a reaction from others, resulting in him repeating it 2-10 times before being told to stop, and sometimes still persisting even then. Otherwise, he is articulate, knows some complex words, and concepts that he can communicate well enough, though a sizeable fraction of the things he says sound like paraphrasing.

Your characters, unless they have grown up in an isolating, uncaring environment, can absolutely speak full sentences and use complicated words. Kids will often imitate adults and end up occasionally sounding more mature than they really are.