I just tied whole oats up in a muslin with a hair bobble and left it under running water. Made the bath milky/creamy and soothing but then there weren’t bits of oats floating in it. Seen other people use tights or socks as well. Not sure where you’re based but we used Poxclin mousse for our LG which was great, and over 1 they can have piriton to help with the itching. Cut their nails as short as you can asap so that when they do scratch the risk of infection is lower.
Calamine lotion helps with the itching. When my daughter had chickenpox, we dabbed it on her spots to ease the discomfort.