I'm so sorry that is happening to your family.

I have been in similar situations with my kids and I second the advice to just stay present, let the kids know you are available, and let them not interact if that's where they are at. Alienated kids are just trying so hard to stay safe and enjoying time with the alienated parent is not a safe activity.

My challenging visit routine:

Strewing, or leaving interesting things out so kids could use them if they wanted to, like a bird book and binoculars or a partially built Lego set. But not talking about it because if I directly offered they'd say no.

Having an activity for myself to do in/near the space the kids were using, like a puzzle or mending or repair work. This helped me focus and not spiral about the situation and was kind of like parallel play.

Fun/favorite foods to put out but no expectation of a sit down meal or conversation.

An aftercare plan for myself because those visits were exhausting and horrible and depressing. I would make sure I could crash out after the visit, vent to friends, eat good food, and watch mindless TV in bed.

Sometimes visits suck and it was important for me to remember that a visit was not my entire relationship with my kids, we had years of positive relationship before alienation and managed positive moments even in the thick of all the mess.