Another blessing Joseph will receive is that his people will not be utterly destroyed. For a young person that is a heavy thing to say; you will have influence and your family will not be destroyed. That is a hopeful thing for a young person to hear, especially one in Joseph’s situation.
If he is eight to ten years old, he has seen his older brothers quarrel. He was probably fairly traumatized by what happened on the ship. His parents are old. They have just landed. The family knew how to survive in Arabia, but they are in a completely new environment. This would be enough to cause doubt, insecurity, and tears in any young child.
In 2 Nephi 3 Joseph is promised four times that his seed will not be destroyed. How can Lehi be so confident of that? He knows that these were promises given to Joseph in Egypt. Lehi sees himself as carrying much of the burden of ensuring that the tribe of Joseph goes forward. He is passing some of that burden to young Joseph, in effect saying to him: "you are going to be the one through whom these promises are kept!" There is also reassurance in that these are old promises and the Lord has protected us so far, you can count on him to protect you further.