All 4 in window seats, one behind the other, assuming the kids are independent and won't need much parental interaction.

Or, if they need a higher level of parent contact, put all 4 in the two middle seats, one pair behind the other, like this:

Row 1: Left parent 1, Right child 1
Row 2: Left child 2, Right parent 2

That way, each parent can talk to the kid next to them, but also get up to help/interact with the kid directly behind or in front. It's hard to access passengers seated on the aisle on the other side of the plane as you have to go through the galley, which is often full of bustling FAs. And the window is pretty useless on an 18 hour flight.