John Gill Commentary Deuteronomy 11:21

"that your days may be multiplied, and the days of your children, in the land which Jehovah sware unto your fathers to give them, as the days of the heavens above the earth." — Deuteronomy 11:21 (ASV)

That your days may be multiplied
Long life being a very desirable blessing, and which is promised to those that obey and keep the law; see (Deuteronomy 30:19Deuteronomy 30:20) (Psalms 91:16)

and the days of your children ;
which are dear to parents, and the continuance of whose lives, next to their own, is most desirable, yes, as desirable as their own; and especially it is desirable that they might have a posterity descending from them, to enjoy for ever their estates and possessions; as it was to the people of Israel, that they might have a seed always to dwell

in the land which the Lord swore unto your fathers to give them ;
the land of Canaan, so often spoken of as the promise, oath, and gift of God:

as the days of heaven upon the earth ;
that is, as long as the heavens and the earth shall be, and the one shall be over the other, as they will be to the end of time.