No. You can start with the simple aspect that OSHA addresses A-weighted, slow response levels, so not 90dB and 115dB but 90dB(SPL, A-weighted, slow response) and 115dB(SPL, A-weighted, slow response). More important, OSHA only addresses 8 hour periods and the referenced chart is indicating the associated levels and periods to be used in calculating the total 8 hour dose when you have varying levels over that 8 hour period (and which is why a dosimeter is really the only appropriate tool for such applications). In other words, you have to comply with all the criteria for longer periods including the total 8 hour dose, which with 115dBA for 15 minutes essentially means you used up all the allowable exposure in 15 minutes and must have no significant exposure for the rest of the 8 hour measurement period. Also note that chart is for the 90dBA exposure action levels and not for the 85dBA exposure associated with having to initiate a hearing conservation program. And finally, nothing in OSHA restricts the source levels, it is all about the exposure levels and active options to reduce exposure such as required use of personal hearing protection along with administrative options such as limiting the time of exposure are common approaches for compliance - and also much more practical in the work place environment for which it is intended to be applied.
I think the gentlemen involved is common in that he misses the big picture in what OSHA and NIOSH are addressing and trying to do, which is addressing long term effects of repeated exposure in the work place. Not only are the attendees not your or the city's employees but you do not have the control over their exposure that an employer has over their employees. You can't make attendees wear hearing protection or limit their exposure time and that is really what OSHA and NIOSH address, not limiting the source levels.
I also don't know how he can expect the "band leaders" to have the bands play 20dB lower in response to children "being present", whatever that means. Maybe Bill can take responsibility for keeping all children a sufficient distance away or making sure they all use hearing protection.
Etymotic Research ETY ER20 plugs are like $9 to $12, probably less in bulk and custom plugs much more, I have no idea where he came up with a $30 number unless maybe he is trying to sell something.
I think Bill may be well intentioned but seems likely to have obtained much of his information from rather biased sources as perhaps shown with references like the "...(sometimes suicidal) persons.."
Good luck!