I always book window+aisle on a 3-abreast configuration. Middle seats are least likely to be filled voluntarily. And, if it does get filled up, I have never had an instance where that person was not happy to take either the window or the aisle.

When my wife and I travel with our two sons in coach on 3+3, we duplicate the strategy above on both sides of the aisle and frequently end up with 6 seats for the four of us. Frankly, this is better than four seats in F in most instances, as it allows the kids to have one seat with a car seat, and another seat without if they want to get up and move about, all the while not occupying either of our laps.

We avoid the center section of wide bodies if at all possible, as that section means that the kids are always close to (if not on) an aisle, which leads to distraction and the potential for them to bother people across the aisle. Putting them in window seats means other passengers are always buffered by one of us and allows them to both look out the window and halves the number of things they can get into