Since you have a 7 year old, they can always ride alone, so technically you won't have any problems though it may not be your preferred grouping.
One nuance of the kids must ride with a parent rule has to do with the separation of "ride vehicles" which for these purposes means an individual unit between couplings. So some rides, like Space Mountain, there are two ride vehicles, each seating 3 in a single file. The under 7 kids must have an adult in the ride vehicle with them even though the second vehicle is right behind the other. On something like 7DMT, the parent can sit directly behind the younger children if need be, as long as they're in the same car (2 rows per car on 7DMT).
One ride that this gets tricky on is Barnstormer, where each car only has one row of two seats. This is where some odd-numbered families have trouble, because it is impossible to seat a parent with more than one child in the same vehicle. Again, since you have a 7 year old, they can ride alone regardless and you can have one parent each with one of the younger kids.