John Gill Commentary Deuteronomy 1:39

"Moreover your little ones, that ye said should be a prey, and your children, that this day have no knowledge of good or evil, they shall go in thither, and unto them will I give it, and they shall possess it." — Deuteronomy 1:39 (ASV)

Moreover, your little ones, which you said should be a prey ,
&c.] To the Amorites, into whose hands they expected to be delivered, (Deuteronomy 1:27) see (Numbers 14:3)

and your children, which in that day had no knowledge between good and
evil ;
not being at years of understanding, and which is a common description of children; it is particularly expressed "in that day", for now they were the very persons Moses was directing his speech to, and relating this history, it being thirty eight years ago when this affair was, so that now they were grown up to years of discretion:

they shall go in there, and to them will I give it, and they shall
possess it :
the relation of which now might serve greatly to encourage their faith, as well as it would be a fulfilment of the promise of the land made to Abraham, Isaac, and Jacob, which was not made of none effect through the unbelief of the Israelites, whose carcasses fell in the wilderness, since their posterity was to enjoy it, and did.