What would your advice be on how to handle this miscommunication issue?

My son, Chris, married Jenna. Back, over Christmas, they asked Corrie to babysit the kids while they were at work. Corrie said, yes. Then it ended up that there was some miscommunication over what times, and what days, and it ended up with some hard feelings. From what I could tell, the miscommunication originated with Jenna, but I withheld my opinion and, apparently, it resolved itself.

Well, then today, this happened to me: Last night Chris texted me, asked if I could watch Zoey and Jaden today from 11 – 5. I said, “Yes.”

Jenna brought the kids over at 10:30 and said, “My mom gets off at 1:00 if you want her to come get the kids after she gets off of work?”
I said, “Well, that’s up to you guys. I’m prepared to watch them until 5:00. I mean, does she want to take them?”
Jenna said, “Yeah, she does.”
I said, “Well, Ok, then. I guess we’ll see your mom sometime after 1:00, if that’s what ya’ll want.”

So Mom shows up at 1:45. She lives about 20 miles from here. I got the impression she didn’t know what was going on either, although she said something about “Frankie’s birthday party today,” (Franky is Jenna’s 25 year old brother) and she didn’t know if they were having a party, but if they were, she knew the kids would want to be there, but she didn’t know if they were having a party or not, but all the stuff was at her house. So, I’m confused. She’s confused.
She takes the kids.

Then I called Chris to let him know that Jenna’s mom did pick the kids up, so they wouldn’t be here at 5:00.
He got kind of angry! He said, “I asked you to watch them until 5:00! Now I have to drive all the way to Smallville to get them, for no reason!”
I said, “Hey! Hey! Jenna just told me this morning… (Reiterate what I said earlier….)”
Then he said something that led me to believe that Jenna suggested that it was somehow my idea! I think he said, “Well, Jenna said you asked if her Mom was going to pick them up when she got off of work!”
I said, “I didn’t initiate this, Chris! I have no idea what time her mother gets off work!”

So now, everyone is a bit at odds, and I’m stuck in the middle.

Jenna sent a text a bit ago, blaming it all on her mom, said her mom was “just confused today.” Her mom is a bit of a wack job, to be sure, but I’m not buying this “It’s all her mom’s fault” business. I don’t know who initiated what, but apparently no one was on the same page, and I think it goes back to Jenna. I got the impression that Chris didn’t even know Gramma was going to pick the kids up.

I can’t just let this slide because it seems to be a reoccurring thing where Jenna is concerned.
So what I want to do is send them both a message telling them that if I agree to plan A, and one of them changes it, I will call them both and hear it from both of their mouths that they agree to the change. If I don’t hear from them both, the plan will not change, and I won’t let anyone else pick the kids up.

I know it’s long, thanks for reading this far if you have.
Do you have any better ideas than a message? I’m afraid that if I try to talk to them in person it will get deflected and garbled…like Chris, didn’t want to hear me out, partly because he was upset.

Or…should I wait until the situation arises and simply do exactly that without saying anything before hand?