This is a tough situation. Poor kids. You could make sure the kids are well fed and dressed as appropriately as possible for school every morning and just be there for them.
Depending on your relationship with the parent the kids live with, perhaps you could offer to get them the clothes they need and be reimbursed for it. Like "I noticed Larla's coat is getting too small for her. I know you're really busy with work, but I'm going to the mall later this week anyway, I'd be happy to pick something up for her". You could also, again depending on your relationship, get in touch with the parent who is suing for custody and get them a list of things the children need. Presumably the other parent will step up. If the parent the kids live with is completely out of touch with what the kids are doing, you could also pick up some stuff from a donation centre (I know my church has one, but I'm assuming there are other places as well) - I'm guessing the kids don't have winter gear at the moment?
Good luck!