I don't do anything to dressed up or specifically Christmassy. I do the same sort of thing they usually wear but a bit nicer.
William will be wearing this:
https://i.ebayimg.com/00/s/MTYwMFgxMDY2/z/GzIAAOxyOa9R4-3e/$(KGrHqN,!mEFHdMlyr,LBR4-3edOCg~~60_35.JPG
He loves to draw and paint so it is fitting for him. He will be wearing it with some nice jeans and some socks with fish on (he loves fish).
Thomas will be wearing this:
https://www.disneystore.co.uk/mickey-mouse-dungarees-set/mp/60728/1500070/
He likes Mickey Mouse and is going to look so cute in it.