When you free range you have the risk of predators. It doesn't matter if they are young chicks or adult chickens, there is always the risk of predators. Young chicks are more vulnerable to certain predators, snakes and small birds of prey for example, but the adults are also vulnerable to predators.
There is nothing to indicate that your broody hens are bad mothers or doing anything wrong. Predators stay alive by catching and eating prey. They are really good at it, their lives depend on it. Chickens are prey animals, that's why they can hatch so many chicks in a season if they follow the old feral routine of hatching several broods a year, many are going to be eaten. Lots of critters eat chicken.
It would help to know what is getting them but don't let them free range. Keep them behind barriers that the predator can't get through. If you knew what the predator was (could be different ones) you could maybe trap or hunt it to remove that one but that is only a temporary solution. There will be others. That's assuming you get the right critter to start with. Barriers are your best protection against predators.
The older and bigger they get the less vulnerable they are to certain predators. You increase their odds of survival if you keep them contained until they are older. It doesn't guarantee a predator won't get them anyway but it reduces the number of predators that might if they are too big for some of them.
Many of us can free range their chickens, including broody hens with chicks, without problems. Others cannot. But they are always going to be at risk. I used to free range mine without problems and then people abandoned dogs in the country. After two attacks from different dogs and the loss of 13 chickens total I put them behind electric netting to protect them.
