I think that if you're with them, you can cut in after a few minutes of chit chat and say, "Dad/Mom I'm sure you can tell me about it in the car, but I think that <name> here probably has a lot of work they need to do, so let's get those papers signed and be on our way."
...of course... that might lead to the other person feeling like they have to say, "Oh no, it's quite alright." Even if they don't mean it...
So, maybe better is, "Dad/Mom, I'm sorry to cut in, but I've got a very busy day ahead of me/have to get home/want to do XYZ, so let's get your papers signed."
Also note that if you make the motions that you're about to leave, sometimes subconsciously other people will "follow" that. They see another person who is in their group gathering their things, standing up, thanking the person, walking to the door... and they just... go with it! So it might help along with cutting in and saying something.