But you don’t need to tell your reader everything. Your main character, just like us, can figure out what others around us are thinking by what they say, their body language, or the faces they make.
I’ve seen manuscripts with three or four points of view – sometimes in the same chapter. I’ve seen POV from an animal: “They ran toward the elephants who were so happy to cool one another with spraying water that they only grinned to see the two little human creatures running nearby.”
Your children’s book needs to be in first person or limited third. Once you pick your main POV character, stick with him or her. If your main character doesn’t know it or see it, then he or she wouldn’t talk about it either.
With older kids, you can perhaps have more than one narrative voice, but it’s a risk. Each voice must be there because it serves a unique purpose.
If you do use more than one point of view, whether in a book for kids or adults, it also needs to be clear exactly which character’s head you are in. Head-hopping in the middle of a chapter is especially confusing. The reader never gets to settle down if first they are the baby, and then the younger brother, and then the older sister, and finally the mother. If you switch POVs, give us at least a page break or ideally a chapter.
(Full disclosure: these are just my opinions. Your mileage may vary.)