I've always liked the Lido and wouldn't mind living there. The central part, as you can see in the photo, is built up and worldly. But the outlying areas on the tips, such as Alberoni, are sparsely populated and quite different. They have cars and busses on the Lido. Water quality isn't great but you can definitely swim; I've done so many times. The water will be warmish and not choppy in early September and the depth is shallow for a good while, thus very suitable for children. There is a public beach and in fact the Venetians consider the Lido their city beach, which is what lido means. Also remember that by law the part of any beach in Italy (including those of exclusive hotels) which is closest to the water is open to the public. Although Italians will not expect foreign tourists to know this and may challenge your presence.
Here is today's article on the Festival from the Huffington Post.
And here is the line-up from the Festival's site.