Deut.1:39

A few verses before, we see how the Israelites didn't trust God and were rebellious, so he tells them that they won't go into the land. They will wander around in the wilderness.
I take it to mean that the children who were alive at that time, who didn't even know right from wrong yet, would be the ones to go into the land. Just an emphasis on the generation that would not see it, and the generation that would. But that's just me, I'm no scholar.

 

Numbers 14:3
3 And wherefore hath the LORD brought us unto this land, to fall by the sword, that our wives and our children should be a prey? were it not better for us to return into Egypt?

It seems that the Israelites were guilty of idolizing their families. God demands to be at the forefront of our every thought, and we cannot put our families comfort or safety above His word. Not that God would have us presumptuously endanger our children, but when God tells you to do something, you just do it. Their children were never in any danger, and it was wrong for them to doubt God and use their children as an excuse to complain, so He allowed them to witness His preservation of the same children who would go into the land after they were gone.