When I flew airlines with assigned seating with our kids (same age spread, but a year older than yours), I took whatever I could get without paying to have them with an adult. Frequently 2+2, especially if flying a plane with 1 side with only 2 seats on that side.

Now we mostly fly Southwest. If the plane doesn't look like it'll be full we each take a kid and do aisle/window in hopes of middle seats staying open. If we know it's a full flight we typically do 3+1, younger child at the window, me, older child on the aisle, and husband across the aisle. My husband is a big man so he needs the aisle. He helps the older child across from him.

Honestly since we have enough tablets to go around flying with the kids is pretty easy these days.