Parents need new phones on AT&T, need help deciding

My parents have been using Nokia candybar phones for the last six years, and I think they're ready for an upgrade. The screens are so scratched that they can barely see them. They are not tech savvy at all, but they want a phone with a good camera and that is capable of browsing Facebook and Instagram. I was thinking of getting them both iPhone 4S's, but those are starting to age and I don't think they'll be fond of the small screen. The Lumia 520 and Moto G are also both on the table, but I'm leaning towards the Lumia because of the simplicity of Windows Phone versus Android (it should be noted that she does have a Samsung 7.0 tablet, but she still struggles with some of the basic elements of android). Their contracts are definitely up, so they could get something new from AT&T or something used online. They want to keep the price below $100 for each phone. Do you guys have any suggestions?