Could be pain. When dogs are in pain, they just don't want to put up with guff from other dogs, even stuff they've always tolerated with good humor.
You know how it is when you have a headache, the goofy coworker and his stupid jokes that you usually tolerate? Now, suddenly, his jokes annoy you. If it's a migraine, you just want to smack him.
Imagine being your dog, in pain, on the floor with the others, and they just look like they're going to come near you. You know it's likely they might bump into you, so you snap at them. Or, maybe they're doing nothing. Maybe they're across the room, but you just want to tell them to stay far away from you, that you're not feeling well. A growl will do the trick.
Orthopedic pain is where I'd look first. But it could be that general "feeling crappy" feeling that comes from systemic issues -- heart, lungs, liver, etc. Your senior just doesn't feel right, feels tired all the time. Might not be anything too serious, just something that needs some tuning up.
Seniors also sometimes lose some of their cognitive functions as they get older. It's not dementia necessarily, but they get confused more easily. It takes longer for brains to figure out what's going on, especially in a fast-moving house where the younger kids are moving at what now seems to be lightning speed. Confusion leads to frustration leads to acting out. In this sort of condition, she might also be feeling jealousy, even if she's never felt it before.
Any of these could be going on. All of them could be going on. It doesn't mean that your dog is very ill, or that much is wrong with her at all. It might mean that she needs a few meds for inflammation in her joints that you may not have noticed until now; or a nice quiet place where she can watch without actually being part of the activity; or a special time when you take her for training and new activities (both help brains stay engaged and stimulated). Special time with you reminds her that she's your special girl and that the others can't intrude on that. Separate trips to the park. Taking a simple obedience class. Separate walks. Drives and nice walks on the beach, just the two of you. (Take lots of picture too, so you can show the others!)
She might also need you to step up and be what I call the Super Alpha. My senior is my alpha, but she trusts me to make sure that the kids don't bug her. She'll relinquish power to me, but no one else. So I have to be on my toes and anticipate.
I have a senior who's in a situation much like I explain above. Overall, she's in great shape, but when she is having a slow day (either physically or mentally), she doesn't want to be bugged. And she lets the kids know ahead of time. When they don't listen (particularly the 3 month old puppy), she reminds them. And my senior/alpha before her was in the same situation (although Grover remained sharp as a tack until she died) when I got Camper, my GSD as a puppy. When Grover died a couple years ago, Zamboni stepped in as the alpha. But being alpha is tough when there's kids and you don't feel great all the time.
I recommend a vet visit. A full exam and a complete blood panel (with thyroid) if you haven't done one in the last six months. Perhaps even xrays if you think there are orthopedic issues. Once you know what's going on, you'll have a better idea of how to manage it.
My guess is that your girl is just getting older. And when we get older, we are entitled to get a little grumpier. That's ok. Give Bear her own space that she goes to (or you send her) where she can observe everything, feel included, but be out of the way (in my house, Zamboni's spot is under the dining room table, and NO ONE is allowed to bug her when she's under there.) Everyone will adjust. The kids will learn when to leave The Boss alone and when it's ok to coax her to come out and play in the sunshine.
If she needs meds, start there. Fish oil, Ester-c and a good multi-vitamin are good not only for joint health/inflammation but also for brain health. Unless there's something specific wrong, it will take you managing things for a little while, until she realizes she can relax and enjoy from the sidelines unless she's up to being a happy participant.
And then, Ruby and Riley's jokes might actually seem funny to her.