My son's most common complaint is that his stomach hurts. He is on many medications and supplements so this is to be expected. I usually offer him rest and cuddles when he complains and a small snack to help with is belly pain. He often complains that his back hurts, but because of the advanced stage of his kidney disease, NSAIDS are not an option. I usually offer him comfort and a soft back rub to help ease his pain. Voiding again sometimes helps.
He has never known what it is like to live without kidney disease and every test and procedure has been approached with the view that this is something that HAS to be done. I usually try to keep a stash of inexpensive toys, he loves little animals, to give him when he has invasive procedures done. He still cries, but he perks up faster with a reward.