Twin parent here — separate them when it starts happening. And then distract, distract, distract, distract. Put enough toys away and out of reach that it’s easy to grab something “novel” (i.e., they haven’t seen it in a month and have forgotten about its existence). “You can’t have that, but you can have THIS!”
I usually made mine play in different parts of the room and that was enough to keep them apart. There are a lot of kinds of gates on the market, find something that’ll work in her house.
It may or may not be possible to find something punitive that really gets his attention.
Taking away toys is my favorite punishment, really. I put stuff on the fridge where it’s out of reach.
Talking? Reasoning? Explaining? Mmmm, not so much.
OK, sometimes I’d escalate to yelling when repeated attempts to communicate failed. I even spanked once, when he yanked her off the rocking horse and she hit the floor. On the whole, though, any kind of hitting on my part merely set a bad example, as they immediately began “spanking” each other. Ay yi yi.
The downside to punishment, if he’s like my son, is he’ll be very confused and hurt that following his natural desire to push and grab everything in sight is somehow worthy of punishment. I think my son felt unloved when he caught hell for it. I had to go out of my way to demonstrate that SHE got in trouble for taking HIS stuff, too. So, don’t attach a lot of meaning to it. He’s just being grabby - encourage your daughter not to label him as a “bully”, even when he’s being one.
Also recognize his triggers. Sometimes it’s particular objects, or being hungry, or tired, or having too much sugary food. Or - if the parents are having a tough time, the kids will often absorb it and act out a day or two later.
The full moon is also bad. My son has been a PITA today.
Another technique - give your granddaughter a minute to figure it out and she’ll bit him back, you can bet. That will teach him cause/effect real quick. She’ll have to go into timeout, just like he does, but…
Emphasize the positive “Hands are for loving”, all that stuff.
Some people advocate making a big fuss over the victim, while giving the perpetrator the cold shoulder.
My daughter also learned to bait-and-switch on toys, and to scream loudly when angry. For a while there I never did get a decent potty break, they’d go bananas whenever I stepped out of the room for just a fricking minute.
Another thing, give him an alternative besides biting. Is he verbal? Does he know any sign language?
My daughter bites when she’s really mad – but I quickly learned to recognize the sound of an escalating fight and catch them before she chomps him.
At age 2 I think I just separated them and settled the squabble (or, went ballistic, depending on how I was feeling), but now at nearly 4 they know how to argue and negotiate (it works about 65% of the time). How we got from there to here was just a LOT of days of trial and error.
Louise Bates Ames wrote a WONDERFUL series on child development, published in like 1986 and still on the shelf at my Borders. Really helped me. I’m SO GRATEFUL that I’m past 2.5 and 3.5 because each of those periods (about 4 months long) was absolutely hellish over here. Daycare, preschool, relatives and babysitters are the best option. And rum.