Kid's don't just magically wake up one day and start acting polite. It takes practice and discipline. If you admit they are being loud around people who are trying to have a conversation, you need to be the one who steps up and fixes the problem.
My grandmother also used hearing aids and she explained that they aren't like having your old ears back. They pick up everything and when someone is talking to you, the sounds of the children in the background can override what you are trying to say to him. If he turns up the device, it only makes everything louder, but it doesn't solve the problem of singling out your voice in the mess of noise.
Taking this issue into consideration, you should train your kids to behave around grandpa so he isn't left out of the conversation because he can't make out anything you're trying to say, just so you can let your kids do whatever they want.
I feel for you though. My five year old is loud and her motor mouth starts up at 7am and doesn't wind down until she's in bed at night. We've practiced playing the quiet game though. And I've discovered activities that will keep her occupied and quiet for small stretches of time.
Get yourself a "quiet baby" bag of tricks and whip them ALL out when it's time to visit grandpa. When my kid drinks a sippy cup of liquid, it takes her a good ten minutes to finish the whole thing. It acts like a cork and keeps her quiet. Sippy done, time to switch to another quiet activity. And so on and so forth until your visit is done. Then they can ramp up again after you get back home. They won't be deprived or wounded if you force them to be quiet for an hour.
Good training for when you want to take them to the movies, or the library, or any other place that requires a lower vocal volume...