Disclaimer: I acknowledge that my views here speculative and anecdotal, but based on my experience living in flood zone and hurricane ravaged area, and taking part in mutual aid responses to natural disasters in other parts of the county. If you believe I am wrong, I am happy to review your source material and learn from it. If you just want to insult me an fight, don't waste your time, you are probably already on my plonk list.
1) The camp, and everyone within miles of the floodprone areas probably do have a NOAA radio, and cell phones with weather apps, etc. The problem is we don't listen to them, particularly when we are sleeping. Many of these devices have alerts that wake you, but we often turn those down or off. Because the way they are designed, the alerts go off pretty much constantly during thunderstorms. They don't do a good job filtering the difference between "patio furniture might blow over 100 miles away" from "Everyone on Oak Street, crawl into your bath tub right now". I know this because I am the paranoid one in my house that stays up for three days straight listening to all of them, looking at a paper map trying to figure out how close the tornados are because the internet is down.
2) The "Cry Wolf Factor" is huge. These folks are exposed to generations of being told "You are in serious peril with great risk of loss and life". This is WILDLY EXCARCEBATED by local and regional news media, as wells as the Weather Channel who feeds the frenzy with exaggerated alarmist rhetoric and even fake footage to keep people from changing the channel. Nearly all the time, the storm passes without incident, and at the celebratory BBQ the discussion is,"They always say that" "It never happens", "They always turn at the last minute" "The highest the water ever got was that tree down there". Does that ean the forecast was inaccurate? Absolutely not. The forecast SHOULD be worse than the actual outcome 90% of the time, because it is PRECAUTIONARY, and weather predictability has limits. But the stress of reacting to the constant "false alarms" wears on people and conditions complacency.
3) This camp, apparently in the same family ownership for 150 years, has generations of storytelling, they might have felt confident in the property history to have said, "We will be fine, the campsites have never flooded, or even "God is watching over us".
4) Circling back to #2, Local Media, both TV and Radio, is evaporating in favor of national syndication. I doubt that flash flood warnings in a sparsely populated area like Kerrville even attracted the attention of the weather channel. So even if the forecast is there, who is talking directly to the people about it? Hopefully local government officials are apprised, but how are they going to get word out to the people when local TV and radio stations are often not even staffed at night. Now many governments have emergency alert systems via texting apps, so to that, circle back to #1. People shut those of because they get tired of awakening for missing grandma and boiled water alerts.
That brings us back to the campground. Ultimately the people responsible for protecting those children is the adults in charge of the camp. With a location in a known flood area, and known flash flood watch in effect since lunchtime, at the very minimum, they needed an emergency evacuation plan and SOMEONE on wide awake all night listening to the NOAA radio for updates.