Family reunions are highly sought after by resorts and hotels. Many have a coordinator who is supposed to work closely with the reunion host. When I had mine last month I had to tell the hotel exactly how many adults and kids were coming. They knew how many people were eating breakfast each day and how many were going to be at the banquet. We also paid for everything five days before arriving. I can't imagine that this resort was unaware of how many kids were coming. They took this family's money and didn't let them use the facility that they paid for.
OK, so what's your guess as to why the pool was closed? It makes no sense that a hotel willing to rent to a group of Black people would only put the pool off limits.
They slept there; they ate there. They probably used other facilities. Why not the pool?
And, BTW, I have booked vacations where something went wrong, and I didn't get everything I expected. There was usually an explanation.