That's an awful experience. Really awful. And totally unnecessary. I know it's work for you but I would email the local council and include a link to this thread on here....they have by-laws which they really should enforce. Really bad that a nice day out was spoiled.
Where I live I report dog shit left on the kerbs daily to our local council, even having actual CCTV of one bloke allowing his dog to shit outside our driveway and leave it, we forwarded this footage to the council and they said dog wardens will come out and also put up notices, a council van did come out and clear up the dog shit, but never did see any signs go up, nor did i see any strangers walking up or down our street, all the council seems to do is clear up the dog mess and don't do anything else.
This old but TRUE story will defo' make you laugh
Back in the early 1980's someone was always allowing a dog to shit right outside our small gate, this happened every day and went on for months with me mam always having to clear it up, one day she'd had enough as the postman trod in it and walked it up to our back door, so got up very early in the morning, be about 4am, no shit outside the gate, so she waited and waited, then she saw a bloke with a big dog and it was him and allowed the dog to shit in the same place, my mam ran out and confronted him, with him becoming very abusive back to my mam, telling her he'll let his dog shit wherever.
Tosser dog owner walked off, but that wasn't the end of it, my mam watched and followed that man at a good distance, right to his very house, noting the door number, she came back and she said to all our family that whenever we need to go to the toilet for a poo, you'll notice a black bucket with some wood on top of it, mam said sit above a bucket and poo in that, replacing the wood cover to keep the smell in, this went on for a week and the smell in our bathroom wasn't good, four in the family all shitting in a black bucket!!
Also the dog shitting carried on outside our house, my mam also picking up that shit and putting that in a bucket at the side of the wall, after a week my mam got the two buckets of shit, boiled the kettle, pouring the boiling water over the poo and mixing it up, she walked to that house of the dog owner and banged heavily on his door, me mam stood back, waiting with the buckets, he answers the door and me mam then throws the shit at the bloke and she shouts....''HERE HAVE YOUR DOG SHIT BACK''' with the shit going inside his doorway and on him with the first bucket, he quickly slams the door, the next bucket went up on his front window......My mam then shouts again...''LET YOUR DOG SHIT OUTSIDE OUR GATE AGAIN AND I'LL DO IT ALL AGAIN''
Bloke shouts out he's calling the Police, my mam shouting back...''WELL BLOODY CALL THEM''
We waited for the Police to arrive but they never did, one witness who saw to incident said she's sick & tired of allowing his dog to shit everywhere and he never ever cleaned it up......That bloke never did let his dog shit outside our gate again.