As Renegade said, it is farther across than it looks and is pretty deep on average. If the kids aren't strong swimmers be sure to take fliats ir life jackets. A few people have drowned in the past 5 years.
It is family friendly, but as with other Austin sites be sure the kids know they may see some new and interesting things. A few years ago I was out there with my kids (teen and pre-teen). There was a tall former male that appeared to be halfway through a sex change in a tight bikini. I think it took my son a while after a few conversations to figure that out. Noone was doing anything inappropriate, and that is part of life these days, especially in Austin.