Reading aloud to a parent or teacher has two purposes: to show that the child can decode words properly and to show that the child can read with fluency. If the child is learning English as a second language, showing proper pronunciation is also a purpose.
First decoding. Probably your kindergarten son is at the decoding stage, that is, learning how to link language sounds with letters to form words. If he is at this stage, then yes, he should read aloud. That way you can tell what he knows, what he needs more practice on, and what he needs instruction on.
If you know from previous reading aloud that your third grader has mastered decoding, then your daughter needn’t read aloud for that purpose. You might sit next to her as she reads. If she has questions about pronouncing an unfamiliar word or if she asks about the meaning of a new word, you can help. Occasionally you might ask her to tell you what she has read to be sure she has understood.
If your older child comes to English from a second language, she might be able to pronounce words perfectly yet have no idea what they mean. If so, ask her to underline words she doesn’t know so you can talk about them. If there are context clues, you might help her identify them. With such a child, you should work on vocabulary development.
As for fluency (reading at a normal speed with voice inflection, pauses for punctuation and emotion in the voice) the kindergartener and ESL child should read aloud. Some readers who are at the decoding stage spend so much energy on decoding that they miss the meaning. By listening to how you say a line and then mimicking the way you say it, the child can pull together decoding and fluency.
Your third grader should be able to read fluently within her head. However, if you notice that your child has trouble with comprehension even though she can decode well, then her reading aloud could help you to figure out why. Is she ignoring punctuation and lumping parts of one sentence with another? Is she sliding over longer words without decoding them because she is lazy or in a hurry? Does she have short term memory problems, allowing her to forget the beginning of a sentence or paragraph before she gets to the end? Is her emotional voice flat? Is she missing inferences? Some of this you can tell by listening to her read aloud, and some by asking her about what she has read.
In general, newer readers should read aloud with instruction and monitoring while experienced readers should read silently.