I think there's some confusion about the question.

I think what the OP was asking is if a child could be assigned a seat away from a parent and how the airline addresses this, not, if the parent is seated on one side of a child, how the airline addresses the seat on the other side of the child.

Anyway, I think gobluetwo's reply hit on the issue well. The only thing I would add is to try and have access to an aisle, so you don't have to get around a fellow traveler to take children to the potty, and to orient the mom as follows:

|Aisle| |Youngest Kid||Mom||Middle Kid||Oldest Kid||Fellow Passenger|

or

|Fellow Passenger||Oldest Kid||Aisle||Mom||Youngest Kid||Middle Kid|

etc.

So the oldest kid is the one seated farthest away from mom and closest to a stranger. Youngest kid is seated closest to mom and farthest from a stranger. Aisle access is nearby without having to ask someone to get up so you can get by.

Depending on how far in advance you are booking this, getting 4 seats together may require a few phone calls.

good luck!