At our house, we have a "no grabbing" rule. I works very well 98% of the time. It has even worked surprisingly well in public (with children we don't know from the library or the park).
This is our protocol.
Child 'A' has something that child 'B' wants or was playing with.
Child 'B': May I have that? (while holding their hand out to recieve)
Child 'A': OFTEN gives it up.
if they don't want to ...
Child 'A' is encouraged to respond: You may have it when I am finished with it.
Child 'B': Thank you.
As mom and mediator, I make sure the child 'B' actually gets their turn. Rarely do I have to make child 'A' give it up. For some strange reason, my kids willingly give up that item when the other asks for it. If grabbing is involved the answer is a hearty 'no' with crying and pulling.
This routine seems to work well of us. It has surprised other moms while at the library and park too. If a child we don't know wants something from my child I simply tell them. "At our house, if you would like something someone else has you just say, "May I have that." Try it. Ask him and see what he says." They do and they get it!
I would love to hear if this works for your kids too!
By the way my 3 year old and my 20 month old are doing this! So try not to limit those little guys! These ages are probably the best time to train. Start positive habits now! It also teaches delay of gratification. They will need to wait for the answer and they will need to possibly wait for their turn.