Guessing often means the child is relying on pictures, memory, or context instead of the sound to symbol relationship in words. Children need a solid word attack strategy to rely on.
For your chronic guesser: Cover the picture and ask, "What sounds do we say when we see these letters?" instead of "What word would make sense?" By asking your child which word will make sense, they may come up with a much better word than is in the text, defeating the purpose. It could lead to the child misunderstanding the purpose of words and how they are represented.