Your pumps are paired with each other in a master/slave relationship, that's the "child" thing.
Because your pumps are facing each other on opposite sides of the aquarium, set them to anti-sync. The exception is if the master pump is set to constant power - you'll want them to run in sync if you enable this mode.
You'll have to set the pump intensity based on the setting it's currently in, see
@Potatohead's excellent description or earlier posts in this thread where the differences between "regular" and "ecosmart" modes are represented in graphs.