We had the visit from the behaviourist this morning, the main things were:
She went over 'dog rules' with the children and they're going to be teaching him some new tricks to help build the bond between them.
We need to work on some things to help him develop his self-control - he's already quite good with the cats and has a very good 'leave it' so we're going to help him learn polite greetings for visitors. We all need to control our interactions with him a bit more - so if he gets a bit overenthusiastic we should just ignore him until he calms down again.
She reminded me that I need to praise him when he's doing something right - so if he takes himself off away from the kids when they're being noisy or otherwise annoying I should reinforce that he's done the right thing.
He already has a crate so in addition to reminding the children to leave him alone when he's in there we're going to start throwing in treats when they are anywhere near it, so that if they do end up a bit close one day he won't be so anxious about it. We'll be doing the same thing for any resources he regards as precious.
She was also able to help me with Sonny's over-excitement when we're getting ready to go out, which will make my life much easier - he drives me mad sometimes!
At one point my DS sat on the floor to read a book and Sonny went over to lie next to him, so he obviously wants to be around the children. I think he likes them more when they're less focused on him.
Her feeling was that as Sonny is maturing he's deciding what he will and won't put up with (he's become more assertive with other dogs too) - hopefully we'll be able to show him it's worth putting up with the children

I was really pleased that she didn't write him off as a vicious brute, or me as an irresponsible owner, and I'm optimistic that we can learn from this and make sure Sonny and the children can get on. Thank you all for your kind words and advice, it's very much appreciated.