I'm glad you came to a compromise. I'd try to stick to that until they are at least 6 weeks old.
Granted, chicks are naturally "born outside", but they are also naturally "raised by a mama" outside. Everyone forgets that little detail when they say you are spoiling them. Do they have a mama to keep them warm? I bet the chicks your grandparents raised, in the past, were being raised by a hen. 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. 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.
Good luck!