The best way to set this up is to give each child a can't serve date of "every other week" and stagger the starting week. So first, from the Volunteers pane edit the profile of one of the two children and click the "Add..." button for can't serve times to add a new can't serve time. In the can't serve times screen, use the option thats ays "The first week of every month" but change "the first" to "every other" week. And start it, for example, this week.
Then, edit the other child, and do the same except for the "starting" week; set it for next week or last week. This way each child will only be available on opposite schedules from each other.