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

First of all, unless they are paying you (and Jenna’s mother) it seems to me that they’ve lost sight of the fact that, even though you may love watching the kids, you are doing them a helluva big solid by watching the kids. I know that doesn’t help you with this Q, but hopefully it will give you a better perspective to start from.
I think a message is a good idea, they can’t interrupt a message, and I think the message needs to be written so they can’t argue with content. And, (while I’m feeling overly incensed for you, because even if they are paying you, no one gets to yell at you) they both need to contact you if plans change, as you should not have to track them down and do the dancing.

<dismounts white horse and sheathes sword>

No they aren’t paying me! They offered, and I said No. I did mention that my gutters need to be cleaned out!

OK. I think a message is a good idea too, for the reasons you mentioned @canidmajor. It was just a huge miscommunication and it ain’t gonna happen again, not on my watch.

Well, it may happen again, but at least you’ll be able to say that there is an agreement in place that needs to be honored, then they may get a bit huffy, then it probably won’t happen again. :-)

Why don’t you make sure to only mass text or mass email when you are communicating with Jenna? Include Chris, even in the smallest of conversations, if any decisions at all are being made. I don’t care if it’s what color shirt the kid will wear.
If you communicate by phone, send a follow up text immediately following the phone call, again to both parents, stating what you think you heard the plan to be.
CYA – and let them sort out the rest.

Should I let them know in advance that that’s what my plan is? Send them a note with a head’s up, while all of this is fresh in their minds?

Nah. Just move forward with that as your new communication strategy. Alerting them to your new plan will only cause pointing of fingers, feelings of blame, and irritation at you.
I’d let this one go and butt out of it, and the next time they ask you to plan a babysitting day, respond to both of them as suggested above, with no explanation or calling out of what you are doing.
It’s similar to cc’ing your boss on all communication when you struggle with a co-worker. No need to explain anything.

In the future I would just confirm plans via text to all involved.
That way it’s all documented.

Thanks, @GloPro.
My son came by after work. Here’s the rest of the story. Apparently they both texted Jenna’s mom, telling her not to come get the kids, but she insisted, but probably vaguely. Again, nobody told ME that there was yet another change of plans. So, anyway. I think there was enough emotion and frustration for everyone involved, that no one will be at all upset if I call to verify a change of plans for ALL involved.

There is plenty of good advice above, and little I can add to details. My only insight is to let everyone know you are working on reducing confusion, and then not assign a reason or blame on anyone.

The diplomatic part is to move forward without anyone finding fault or resentment (which to be honest seems to be close to the surface in various people). Be positive and show you are working on helping all to get along well.

^^^^ Agreed! Thanks @zenvelo.

Thanks y’all.

There are 2 approaches that seem reasonable to me. The first is to refuse to babysit, until everyone confirms that they are on the same page. The second and best solution involving your level of inconvenience would be to state that as far as you’re concerned the kids are parked with you, and whichever member of the triumvirate shows up to claim them gets them regardless of previous instructions. It isn’t your job to referee custody.

How exasperating it must be to do a favor and then have to deal with chaos. I would ask Chris to be the communicator in future so there’s no confusion. Jenna may just be a dingbat so that would solve the issue.

LOL @KNOWITALL!
Well, I’m trying to set a good example. I just sent them BOTH a text that said, “I won’t be available for babysitting on Monday until about noon.”

If you all have smartphones, I’d create a group on WhatsApp with those two.

I don’t have a smart phone.

^ Keep it in mind, should you ever get one. Solved some similar problems in my family…

How does it work @longgone?

WhatsApp is nothing more than a (free) messaging service. The ability to create different groups is convenient, though. For instance, you could call a group “babysitting” and have Jenna, Chris, and possibly Jenna’s mum join. Then, all negotiations concerning babysitting dates would be written down, easily accessible for all.

Well, I can create groups on my dumb phone!

Can you? With all the conversations on a specific topic for everyone in the group to see?

No. I can just set them up so that I text certain people. Right now I just have a “Family” group set up.

^ Ah. That’s handy, too!