chicks are naturally "born outside", but they are also naturally "raised by a mama" outside.
Not always. My chicks have been raised for years outside without a mama. As are many, many chicks.
I bet the chicks your grandparents raised, in the past, were being raised by a hen.
Not necessarily. My 96-year old Grandma remembers as a child, her mother incubating chicks in an incubator.
So, since they don't have a hen to "show them the ropes" and keep them warm, it isn't crazy to be a little cautious when it is still early spring and cooler.
Chicks manage to figure things out pretty quickly. Mine are two weeks old this weekend. I put fresh straw down for them the other day, and they went crazy scratching and pecking all by themselves. Not to mention, they figured out the food and water almost immediately after I put them in the coop at 2-3 days old.
That being said, you are in a pretty warm area, so they would be fine with a heat lamp at night for a couple of weeks and then without any added heat after that.
They're a month old. They're pretty much feathered out now, and will be just fine. Especially with the temps the OP is having right now.
All that being said, OP - you need to get that coop secured and get those babies out of the house for your grandparents' sake. They did you a big favor letting you have them inside as long as they did. They didn't have to do that for you so you need to do as they ask now. It's the right, respectful thing to do.
Good luck!