But your little ones, which ye said should be a prey, them will I bring in, and they shall know the land which ye have despised, Numbers 14:31.
In late Modern English (what we speak today) the word "should" is spoken as a moral imperative. "You should get a job". By saying it this way, it means that it is my opinion that you have a need to obtain employment. I didn't say, "You can get a job", or "you will get a job". I expressed it in such a way as to let you know my opinion, the right thing for you to do would be to get a job.
If we were to apply the same reckoning to Early Modern English, the opening verse would be ridiculous. It would be as if the children of Israel felt that it was morally right for their little ones to be a prey. "Hey Lord, we think it would be best if our children became a prey. You should make them that."
Once again, we are reminded of the exactness of the Early Modern English wrought by the King James Translators to convey to us exactly what the Holy Ghost meant when he first expressed certain thoughts in Hebrew and in Greek.
In Early Modern English, "should" is not an imperative, it is not expressing an opinion as to what the speaker desires the hearer to do. It is expressing either a hypothetical notion or a probable notion.
Hypothetical: if it should rain porridge, he would want his dish.
Probable: If I go out in this rain it should be to my death.
What the children of Israel said in Numbers 14:31 is that in their opinion, God's current plan would guarantee that their children would be in danger (should be a prey).
As an example look at Romans 6:3-5, we see both the present tense of "should" and its future tense "shall".
Romans 6:3 Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death?
Romans 6:4 Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.
Romans 6:5 For if we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection:
In verse 4 we are told that the expectation for the believer is that he should walk in newness of life. Here we have "should" used as a natural expectation. If we have been baptized into his death, it is expected that we walk in newness of life. That is a present tense rendering. In verse 5 we see it in the future tense. We shall be also in the likeness of his resurrection. That is not stating an unchangeable future event. It is stating a future expectation. The next few chapters of Romans explains to the believer how to walk in the likeness of his resurrection.
Shall is the future tense of should. We often think of it as an inevitability, indeed it is inevitable if God says it. In your King James Bible if God says that he shall do something, you can bank on it. That inevitability is contained in the nature of God unlike the word shall being used in relation to human expectation.