quite confused

This year I have been spending a lot of time reading in the old testament. Reading there, always makes me so appreciative of the saving Grace of Jesus Christ.
As I read about the sins of the Old Testament leaders like Moses and Saul and David ect. I am so shocked at the often times severe punishment that God gives for sin. It makes me realize how very serious God is about sin and how much he hates it. There is a lot about the Old Testament that I don't understand though in regards to how much changed once God's grace came through Christ.
One example is the issue of sins of the father. Do our children still have to suffer because of sin that we have committed or was that washed away when Jesus died for our sins? Does it matter if the sins were committed before we were saved?? Does God still "punish" us for our sins even if we have already repented of them and been forgiven. Just because we have been forgiven, does not clear us from the consequences of our actions. Again, does it make a difference if the sin was committed before or after we were saved??
Tonight I was reading in Samuel, and noticed that a lot of times when Samuel sins he realizes his sin and cries out to the Lord for forgiveness and Mercy, but God still gives him a punishment for his actions or even just the actions of his people. For example in 2 Samuel 24 David was conscience-stricken after he had counted the fighting men, and he said to the Lord, “I have sinned greatly in what I have done. Now, O Lord, I beg you, take away the guilt of your servant. I have done a very foolish thing.”
Before David got up the next morning, the word of the Lord had come to Gad the prophet, David's seer:
“Go and tell David, ‘This is what the Lord says: I am giving you three options. Choose one of them for me to carry out against you.’ ”
God then gives him 3 options for his punishment, and in the end sent a plague on Israel and killed 70,000 men!!! For a sin DAVID committed!!! That seems so harsh! And how awful to think that someone else would have to pay for MY sins.

So, it brings me back to my thankfulness for God's great mercy, but still makes me worried a bit to know if I or others still have to pay for the consequences of my actions.

And here is one more little side note question I would love to know the answer to if anyone knows.....the sin that David committed was counting his army “Go throughout the tribes of Israel from Dan to Beersheba and enroll the fighting men, so that I may know how many there are.” 2Samuel 24:2
Why was that a sin??? and now that I just read verse 1 again it says Again the anger of the Lord burned against Israel, and he incited David against them, saying, “Go and take a census of Israel and Judah.”
Isn't that God telling David to go count them???? Now I am really getting myself confused! Why would God punish him for it if he told him to do it????