Um, she kicked her shoes off in the room Im pretty sure of, before she rescued Eustace. so then she must have got a new pair on board Dawn Treader, and got them back when they returned home.
she kicked them off when her, Edmund, and Eustace fell into the water, and as Caspians shoes were too big for her, she went barefoot for a bit, until, i believe, she bought some on the Lone Islands.
Yeah, Lucy got a lot of stuff in the Lone Islands, shoes included. She tried on Caspian's before they got to the Lone Islands, but they were way too big. It doesn't say that she changed back to her other old ones that she kicked off in the sea at the end, so she may have been allowed to take the new ones home with her.