KennDodd I know you’ve had a bad experience in the past but I think you’re worrying a lot about a trip that isn’t for quite some time yet.
Did you say you’re flying into SFO? DH flies through there a lot with work and always does immigration there when he comes back from Asia, he doesn’t have global entry and he’s always been processed politely. And there are so many others on this thread that fly to the US frequently and haven’t had a bad experience. I know it’s happen to you before but it’s not a given that you’re going to be yelled at.
Maybe you can prepare the kids and tell them that the agent is trying to spot bad guys and has a lot of people to get through so they might ask them some questions and they might be a bit grumpy. BIL says this to his kids and I thought it was quite a good idea.
Then focus on planning the fun stuff rather than worrying about the border which, not including the queuing, is literally 5 minutes of your holiday.