Of course most 15/16 will drink at a party no parent is that deluded. But it is deluded to think offering a couple of Smirnoff Ices per child is magically going to stop smuggling in of more drink, it encourages it.
16 year olds drinking at parties is not new, but "cool" parents facilitating it for large number of children with no regard to how they are going to manage such large numbers of children under the influence of alcohol was, to me any way, very new.
No adult should be giving 15/16 year old children alcohol without their parents consent. The child's parent has probably already given them a quantity of alcohol they are happy for their child to consume at a party and another adult thinks it is ok to give them more and it is not their responsibility if that child takes both?
No adult should be facilitating 25-30 children under the influence of alcohol without adequate supervision. Most parents disappear off to their bedroom when their children have these large parties (it isn't "cool" for mum and dad to hang around) and have no idea how much alcohol is being consumed until the drama they have fuelled starts. Then they have the gall to either blame the naïve child for drinking too much (under their watch), or ask the child's parent, who had no idea their child will being given extra alcohol by another adult, to pay for the damage.
The "cool" parents may be trope but it is also very fitting, and descriptive for many irresponsible parents of teenagers. Unfortunately the irresponsibility in these cases extends to other children.