Dog obedience help

We got a rescue on Sunday. A 3-4 year old terrier mix per the vet. Rescue said shih tzu/poodle mix. The wife and I go to the rescue to meet the dog and get a feel for how it would fit in our family. Sweet dog think it will be fine. We have 3 kids, 13, 9, 5. Go in great detail with the kids how we need to take our time with the dog to let him learn new surroundings and our family and we need to learn about his temperament. Kids meet the dog Sunday afternoon all is well that night kids are going to bed and go tell the dog goodnight and he growls at them. I correct the dog and tell the kids to back away. Monday morning all is well the Monday evening same thing. I correct again. Same scenario Tuesday. Tonight happens again so I flip the dog on his back and have the 2 oldest kneel over him while I have them look him in the eye. I have obedience training in the process of being set up. Any advice on something else to do before the trainer get started? I can't allow the dog to growl and get snippy with my kids.