First of all, this is YOUR decision. Some women want everyone possible in the room with them; others don't want the mob. It's intensely personal and there's a lot going on.
Some mothers really want to be there; others don't. So part of this involves both potential grandmothers' reaction. It's also a rule in OB-GYN circles that all babies are born at 5 am, so there's the time factor to think about if you're not going with induced labor/c-section.
That said, keep the first sentence in mind. YOU make the decision, and YOU are the ones to call and say "it's time".
I'd first ask what the moms' feelings are. That will have some impact on this. If they don't want to be there, and you don't want them there, then it's easy. "We'll call you when the baby's here". If they want to be there, and you want them there, then it's easy. "We'll call you when she goes into labor." The only hard one, then, is Mom wants to be there and you don't want her there.
Depending on the relationship, that can be easy or hard. I'll give the worst-case scenario: it's Mom's little girl and she feels she has to be intimately involved in everything, and your wife wants her to keep her distance. Then I'd say something like "Having a baby is pretty stressful for us; we don't know when it will arrive and we're going to have a lot going on at the time. So please forgive us when we say it's probably not going to work out to have you there."
Yes, there may be hurt feelings, but quite often those subside when then meet the grandchild. Also, women understand how taxing childbirth is, so most women will support whatever decision you make.
Lastly: what's normal for who's in the delivery room? It depends on the country and the woman. I've known people who had seemingly everyone in the room, and I've known people who insisted it be just them. I'd ask the facility where you plan to deliver. Trust me, any question you ask will be something that they've heard before and are ready to answer. They'd rather answer a question you consider silly than have you do something silly because you were embarrassed.