I saw a dog training episode where the trainer told the owners to take the dog to a vet 1st to rule out any underlying medical issues that might be causing him pain, making him aggressive to his owners. I'm not sure if that's the case for your guy bc he's only doing that to your son & daughter?
Here's a good link that describes all the reasons why a dog can be/ get aggressive:
Aggression in Dogs | ASPCA
Thank you for rescuing, hope it'll all work out!