The switching is still relatively new so you as their parent need to be checking in with them before you drive off when picking them up. They aren't in the habit yet, these things take time. A little hand holding in the beginning here won't hurt anything. The other thing is their mother should be doing the same. Honestly it would be a really big thing if you could suck it up and have a grown up co-parenting conversation about this and get on the same page. The sooner you guys can establish a united front on things the easier it's going to be as the kids get older and the problems get bigger. Now based on my personal experience I know this isn't always possible through no fault of your own but you should at least try.
In my case both my ex and my H's ex were unreliable at best. We couldn't count on our exes to run through the check list before they picked them up or dropped them off so before they left us or if they got dropped off instead we'd run through the list so we could make sure they had everything and if they didn't prioritize pick up/drop off based on the need and how big of a fight one of our exes would make it to come and get it/drop it off.
As your kids aren't babies it would behoove you to have the conversation R2C's spoke about, that they are in charge of their own things and that they need to get better at this as it's going to get more and more complicated as they get older. They will need to transfer more and more things from one house to other: projects, laptops, extracurricular uniforms, phone chargers because they lost the one you or mom bought for that house, library books, etc. And eventually there will come a point where you have to say too bad so sad OR nope, see if mom will bring it to you because I already took you to go pick up something once/that's a want not a need/I've taken you back to get something every time you've been with me for the last 5 times you've been here/mom and I have been taking you to go get things or bringing you things for weeks now, we're done.
As to what ovr said, giving your kids grace with these things doesn't make you or them soft. It makes you understanding. Adults forget things just as often as kids. The difference is they can correct their own mistakes, pay for a solution or make the decision themselves as to if it's something worthwhile or not. But like everything else in parenting there has to be a limit. You just need to find your sweet spot. Until then guide them. Don't make the picking things up about your feelings about seeing your ex, make it about them learning a new routine and new responsibilities. You don't have to disregard your feelings here, but you should consider having them take a back seat. One because maybe it'll make this easier if you're thinking about how the kids are feeling about this and two because while kids are resilient, having to go from one house to the other is hard on them until it's become their new normal fully. Upsetting you because they forgot something is only going to make those big feelings even bigger for those kids. Like all this stuff you have to put their emotional needs before yours, and maybe that's the exact distraction you need.
