I forgot my manners one holiday dinner at my aunt's and dove right in. We skipped grace that year. Apparently no one wanted to interrupt me.

Unexpressed gratitude is like having a present and not opening it. If someone wants to express gratitude, why not let them. If they begin to proselytize, then it's my house and I can stop it.

yes because the world will end if you say a prayer. it's not us vs. them.

If they accept your role and you feel like being gracious, then let them pray.
If someone wants to make it a battle, let them know they are on foreign turf. IMO

I'd be a gracious host and let them get their nonsensical 10 second prayer out of the way. I wouldn't bow my head or say amen, but I'll let them have their moment. Now if they start speaking in tongues, then it's a whole new ballgame. Also, I might have to borrow some money off these chumps someday, so....

That all depends if you want war or peace. You can make it fun though, when they say grace and thank the lord you can add to thank the cow that gave the milk, the farmer that grew the potatoes, the illegal immigrants who did the hard slog and picked the vegetables because the Americans do not want to do those hard jobs for pittance. How about that angle of it all?

Why give in? It's your house so don't let them have the high ground in this. Also, imagine over 2000 years ago with no refrigeration and people praying, thanking god for the food, and asking that it be used "for the nourishment of their bodies." I know exactly what that means under the circumstances.
Todays believer goes right into how god controls everything and without him you would have no food. Wrong! Just totally wrong on the "grace" idea. It should be self evident.

Tell them that in your house you do not say grace prior to eating, but it is fine if they choose to privately thank their god for whatever they like - after all, their god knows all of their thoughts, so they have no need to say anything out loud.

Your house, your rules

If it's a big contingent it might be easier to serve the food buffet style. Then arrange to be at the start of the line and just start eating as soon as you have your food. Situation solved without any confrontation. But if you want to fight all weekend, then just don't give them a chance to start grace before you chow down.

You should be a gracious host.
Grace at dinner is ok. You don't have to say amen or anything.