When the narrator says that the kids had a very big day in store she means that

a lot was coming their way

Explanation:

there aren't answer choices so this is basically what that means

the anwser is went creeping down the river again, across the wide lonely marshes to the sea.

i longed to go with them out into the world to seek my fortune in foreign lands—africa, india, china and peru!

and, dreaming of the lands i had never seen, i'd sit on there, watching till they were out of sight.