I agree with MIcreek's analysis.... each state has unique laws that govern this set of liability issues for juniors. I think you are wise to just copy the states paperwork.
Who makes the call and what is the basis for the call is complicated in the case of a regatta with adult and junior teams competing in the same class. In general this is a question that you might not like the answer. I raised your very issue with the US championships which now has no age limit (unlike other championship events) and quickly closed the lid on this box because I don't predict a good outcome.
The OA is running one race, if they allow entries for junior teams then IMO you have raised the bar to wind and conditions that the PRO would use for a junior regatta. I don't believe you can send some but not all competitors in.
On the other hand, if the OA is running a junior and adult class, I believe the PRO could send the junior class in at their discretion. I see this issue as similar to the PRO running a race for monohull keel boats using the PHRF Saftey standards and a Beach Cat. These are apple and oranges and unless the PRO has lots of experience.... he would be unprepared to make this call or even expect that he should consider the issue.
I believe the pragmatic answer is for the PRO or their representative to have a heart to heart with the junior teams and ask them to consider sitting this one out if it becomes an issue. It is a protest and appeal waiting to happen tho!
As an OA.... I have always viewed the issue as one where the adults accept the lower wind standards for the good of the sport (eg... including the up and coming junior teams). The PRO has to manage the event and if they pull the plug... they would be wise to not fully explain their reasoning... afterall... it is the OA who has accepted junior entries.
One way to handle this is to enter the junior teams into their own class and then allow them to be scored in the adult class as a bonus. If the conditions warrant... the junior class is sent in .... They just take a DNS for the adult regatta.
As always... YMMV.