I have a Rave Hydro tube that is great for small kids. Looks like a race boat and has a scoop on the back that shoots a big rooster tail.
I guess they don't make it any more, but there is a newer version here:
http://www.skitube.com/ski-tube-bran...droskitube.cfm
It's very stable, can't really make it slide or do anything, but for small kids, especially nervous ones, it's been a great way to get them into it.
I just bought, for this year, an HO Switch tube, which is 3 tubes in 1. It's an 84" wide deck tube, or the center section can be removed and used as a 2 person deck tube, while the outer ring and floor can be used as a 3 person "sit in" tube. Looks like fun.
I will verify though, a tower is not a good place to pull a tube from and would recommend you pull from the stern. A high pull will have a tendency to lift up on the tube. Maybe fun for adults, but I wouldn't recommend it for kids. Even for adults or bigger kids, I've always had more luck with a stern pull than a tower pull... Just my $0.02.
...Jim