Oooh no, definitely not Hangouts.... On the few occasions I have been guided to use it, was by spammers. It doesn't have a good reputation.
Whatsapp would be the best option imo. There are a lot more features on it now than there used to be. Like blocking of groups etc.
But the best app for your dc to use would surely be the one that your family uses so everyone is using the same app for the messages/photos etc? You could make a family group, and then as you will be on it, you can see everything that is messaged.