As far as I know, there is no height restriction for the slide. However, only one person at a time may use the slide - a parent cannot ride down with a child on his/her lap.
You can get a child's life vest at Community Hall. The life guard suggested that to my SIL when he noticed that she was standing near the bottom of the slide waiting for my nephew each time he came down. After he got the life vest, she no longer felt the need to actually be in the pool when he used the slide (but of course we were still nearby).
HTH.