All of Qeepsake's accounts allow you to add an unlimited number of journals (one for each of your children). All plans are priced per-account as opposed to per-journal. So Qeepsake plans cost the same for a parent of one as it is for a parent of five.
Related to that, when you see that Qeepsake Plus offers 2 questions per day, this means 2 questions per day in total, as opposed to 2 questions per day for EACH of your children.
Every time Qeepsake asks you a question it will alternate to the next child (if you have more than one journal).
Thus, if you had Qeepsake Plus (2 questions per day) with 4 journals, you would receive questions in this fashion: