Here is a quick discussion on Hula Hoops
You can go down to the cheap $ shops and get a hula hoop that has a join, stapled together and has limited life or search for a decent one. I searched “Hula Hoop no join” or Hula Hoop Fused Joint”. I suspect that these are used in gymnastics and interestingly were not much more expensive, studier and last a long time!
60cm was the size I chose as what you will find is the smaller you choose the more the children will keep their legs closer together as they go through! You may call it streamline or torpedo (which is a sill term to use now days as how many young kids would know what a torpedo is anyway). Regardless that is what we want and thought the 45cm ones may be too narrow (especially as sometimes I use for adults).
I also went for the hollow ones (which were cheaper than the solid core) simply for the air inside and knew they would float (and were actually cheaper than the solid core).
Lastly consider the colour. I began teaching totally blind & legally blind children to swim. I therefore knew that contrast is important for many with sight issues so believed the red is too close to a black line on the base of a pool, blue against a blue pool could present an issue so I went with the Yellow. Others may have a more professional explanation but that was my logic!!