She just needs to grow up some, and have constant correction. Sometimes a little "timeout" in the crate can help settle them down.
Finn, 17 weeks now, spent a few weeks dragging a 3' leash around the house for the same reason. He would jump up like a rabbit and bite at us. He still jumps when he gets super excited, but not like he did. OMG!!!
As odd as it sounds, you may want to consider a padded harness for her. When she starts to jump up, you can safely either pick her up just enough to get those front feet off the ground, or hold the leash on the floor so that she self corrects herself. The leash needs to be attached. Mostly though, you just have to let them grow up a bit.
You also have to convince the two grand kids, and everyone else, to not let her jump up. At 12 weeks old she probably hitting them in the stomach, or chest, with her paws. In another 4 weeks those paws will be at face and eye level for them. She'll stand taller than the 6 year old within 6 weeks.
Jumping up sucks!
You'll never beat a lesson into a dog, you just beat desire out.