We too live across the street from Grandma and Grandpa. Both of my kids do this when its time to leave. My niece is infamous for it. What I tell them and my niece is that if we don't leave with smiles, then mom and Dad will say no the next time you want to come, because they don't like it when you cry about going home.
I do NOT agree with your Granddaughter slapping her kids. Have I gotten mad at my kids over not going home? Yes. Have I spanked them once or twice for not following directions and spending 20 mins with tears to get shoes and jackets on? Yes, but I don't slap them. That is just wrong.
While it isn't your problem, you can be the solution. Talk to the 3y. Tell her how it hurts your heart to hear her cry about going home. That it also make Mom sad about her not wanting to go home. That when she cries, you cry, and you don't like to cry. So even tho you and the 3y know that she really wants to stay, and that you love having her there, to make everyone happy, she needs to say 'yes, mommy' when it's time to leave. Create a special saying, wink, handshake, hug, etc. Since you are so close, you can end with a 'see you at the same time tomorrow...'
Another thing to try, is since you live across the street, have you tried walking them home instead of Mom picking them up? You can sing the song 'Here we go home from Grandma's house'. Skipping is optional. :)