I take my kids every year and didn't bother to check further details once the day was announced. We arrived just after one to deflated bouncy castles and emptiness. Why the 1PM finish??
Does seem a bit early. Was until 3pm last year. https://www.barnsleyfc.co.uk/news/2016/july/2016-open-day-key-information-and-timings/
You would have thought the Club might have posted a message on the official site reminding fans that this year unlike those in the recent past the event would end at 1pm. I would imagine there could have been others who like your kids and yourself ended up being disappointed.
I presume it's because it fizzles out around that time anyway. I arrived at 1pm last year as I'd arranged to meet my dad then but there was literally nothing happening at all and around 20 people stood about (in fairness it was raining).