If the wall was a perimeter, then different standards apply (as passersby might catch themselves).
Within your own property, it's different, so if he had spikes on a toilet flower planter in the middle of a garden then there would probably have been no action. You do have to maintain property to a reasonably safe standard for visitors (particularly places where callers would be expected such as paths to the front door and around a driveway), but it sounds as if the DC are trespassing beyond that. As they are not causing damage, and may be under 10, then it's highly unlikely any action would be taken against them.
Are the parent of one of the DC, then I think the best action is to ban them from entering that person's property and restrict playing-out privilege as sanction if they don't.
If it's your neighbour (shitty, I agree) I doubt there's much you can do unless people are defecating there frequently enough for the smell to be causing an environmental nuisance. (On first reading, I'd though it was old/discarded/filthy, not current use. Though outdoor netties aren't illegal, even if (like composting loos) not attached to a sewer; but if it stinks so it causes a nuisance then action can be taken to enforce better disposal.
And of course if defecating in a front yard, then I think there's an offence of outraging public decency