In July it's unlikely there will be any beaches you can body surf. There simply isn't any real surf that time of year. You can pull a tube with almost anything but the normal 9 to 15hp dinghy is not going to make it that interesting. In addition it's extremely bad form to pull tubes through a anchorage or anything else for that matter.
I think you will be very surprised at much fun they have without the items you mention. I have never taken a single child down there that did not have a great time. If you want to add something interesting have them get their Scuba certification down there. Do the pool and classroom work at home before arriving.
G