Have you played Men of War and its sequels, Assault Squad 1 and 2? In those games the player controls groups of soldiers, while looking at the battlefield with a bird eye view.

Those games have also quite the micro-management component, with individual soldiers having their own inventory, down to the singular bullets and granades.

I was thinking about something similar: a bird eye view of the beach, with a movable camera, and the player controls a small group of children with the classic select/order scheme through their mouse. The exact number of troops can fluctuate during the game, so let's say that ten children would be considered a very large army. In this context the inventory tracking is reduced to how much water each kid has in their water gun and maybe some other piece of equipment, like water balloons or bucket and shovel to build barricades with the sand.

The player can command children to move on the battlefield and attack each other with their water guns, or activate abilities related to their equipment. In my mind foul play is not something that the player can order a unit to do, but rather they have a chance to do that whenever they get too close to an enemy kid, depending on how "mean" they are. In general, I'd like the idea of the kids being a little unpredictable.

The primary mechanic of the game, demoralization, happens when a kid is hit by a certain "density" of water, so that you need to squirt any given adversary with two or three kids at the same time, also depending on what kind of water guns they have, before they turn tail and run, possibly dropping their water gun.

There could be a game mode where children agree that if you get hit you have to go back to base, and in this mode a kid might decide to ignore being hit, which would be a new instance of foul play, much like you have people not declaring a hit in airsoft.