Matthew Henry Commentary Numbers 14:20-35

"And Jehovah said, I have pardoned according to thy word: but in very deed, as I live, and as all the earth shall be filled with the glory of Jehovah; because all those men that have seen my glory, and my signs, which I wrought in Egypt and in the wilderness, yet have tempted me these ten times, and have not hearkened to my voice; surely they shall not see the land which I sware unto their fathers, neither shall any of them that despised me see it: but my servant Caleb, because he had another spirit with him, and hath followed me fully, him will I bring into the land whereinto he went; and his seed shall possess it. Now the Amalekite and the Canaanite dwell in the valley: to-morrow turn ye, and get you into the wilderness by the way to the Red Sea. And Jehovah spake unto Moses and unto Aaron, saying, How long [shall I bear] with this evil congregation, that murmur against me? I have heard the murmurings of the children of Israel, which they murmur against me. Say unto them, As I live, saith Jehovah, surely as ye have spoken in mine ears, so will I do to you: your dead bodies shall fall in this wilderness; and all that were numbered of you, according to your whole number, from twenty years old and upward, that have murmured against me, surely ye shall not come into the land, concerning which I sware that I would make you dwell therein, save Caleb the son of Jephunneh, and Joshua the son of Nun. But your little ones, that ye said should be a prey, them will I bring in, and they shall know the land which ye have rejected. But as for you, your dead bodies shall fall in this wilderness. And your children shall be wanderers in the wilderness forty years, and shall bear your whoredoms, until your dead bodies be consumed in the wilderness. After the number of the days in which ye spied out the land, even forty days, for every day a year, shall ye bear your iniquities, even forty years, and ye shall know my alienation. I, Jehovah, have spoken, surely this will I do unto all this evil congregation, that are gathered together against me: in this wilderness they shall be consumed, and there they shall die." — Numbers 14:20-35 (ASV)

The Lord granted Moses' prayer insofar as He did not immediately destroy the congregation. But disbelief in the promise forbids the benefit. Those who despise the pleasant land will be shut out of it. The promise of God would be fulfilled to their children.

They wished to die in the wilderness; God made their sin their ruin, took them at their word, and their carcasses fell in the wilderness. They were made to groan under the burden of their own sin, which was too heavy for them to bear. You will know my breach of promise—both its causes (that it is brought about by your sin, for God never leaves anyone until they first leave Him) and its consequences, which will bring about your ruin.

But your little ones, now under twenty years old, which you, in your unbelief, said should be a prey, them will I bring in. God will let them know that He can make a distinction between the guilty and the innocent, and cut them off without touching their children. Thus God would not utterly take away His loving kindness.