Just got back and in short, the answer is no. The Menehune Bridge is for 48" and under and they enforce that (no adults on the bridge at all to assist little ones). The two big slides did not have any restrictions, with one exception I'll mention below. They allowed me to stand at the bottom to retrieve my small children who were in life jackets. They let my son (age 4, 30lbs soaking wet, 39" height) ride the tube slide in his own tube. My 4 year olds had a blast on the slides and LOVED that they were old enough to ride alone.
I did see one lady with a very young child be told the baby could not ride. I would guess the baby was ~10 months old and was wearing an infant life jacket. I could barely overhear, but the life guard and then the supervisor *I think* said the child needed to be able to sit up and hold the handles on his/her own to ride. So, no specific height restriction, but certainly there is some point where a child is too young. I saw others younger than my 4 year olds ride without question.
Hope this helps!