A stack of cheap bath towels (in nice colors, why not?) by the door, and a roll of paper towels, with a spray bottle with fresh water. Then your niece can put a towel on the floor for the pups to stand on, and spray and dry their feet. That's what we do in the rainy season (SoCal), and both dogs are used to standing on the towel. We actually washed the dogs' feet every time we came back from a walk in the early days of the pandemic. They got used to lifting each paw, one after the other, so we could wash them. Such sweethearts...