Pool rules are written for safety; you cannot waive a basic tenet of pool safety by allowing "work arounds." Intentionally waiving pool rules can be grounds for lifeguard dismissal at most pools; messing around with safety rules is not tolerated by pool management -- no pool management under which I ever worked (and I worked for several over the years).
A personal floatation device (PFD) is worn based on the individual's weight. A 35lb child's life vest is designed to support 35 lbs. It will have zero value with an adult's weight attached to that child, immediately behind that child dropping off a slide into deep water. The adult has little control over how they come off the slide, there is no way to avoid the adult going underwater there, and absolutely no way to guarantee the adult will let go and the child will stay above water. The rider on a slide needs to be able to control their own body, swim (with a PFD) at least a minimal distance away from the bottom of the slide, and handle themselves in the not-unexpected event of swallowing a mouthful of water and/or going under. There is no waiver language under the sun that would hold up in court if tragedy occurred after a posted pool rule was intentionally ignored or waived by a lifeguard.
I'm all for advocating and making accommodations whenever reasonably possible. But pool safety is not an area that should be up for discussion. Please respect the lifeguards and posted pool rules.