I've stayed (in June) in two hotels with twin beds with my niece that I liked - one was Hotel Atmospheres (great location, small rooms, big bathroom) and Hotel Fabric (larger room, smaller bathroom) - both reviews as well as things to do/see in Paris on my blog - same as user name. I tried to book airbnb in June but was turned down by host and other properties were walk ups and I had an injured foot so back to hotels - September is lovely time to visit - enjoy