This is crazy, James. Firstly it's commonly-held assumption that a "documentary film" is synonymous with Hollywood and that the filmmaker is making a pile of money off the people who appear in it. Reality check: the vast majority of documentaries do not make a profit, barely break even if they are lucky. Secondly, unless someone is being asked to perform specially for the film, or an expert is being asked to do some particular research, it is not normal practice to pay those who appear in documentaries. There may be a very small, nominal fee paid to interviewees, but this is mostly symbolic and connected to a release form.
The way you explain your project makes it clear (a) that the dancers are going on tour at the behest of the company, not at your command (b) they are more than willing for you to document them. This is a good start. It seems that the problem lies with the local union office who are almost certainly in the grip of the Hollywood illusion cited above.
Firstly, with the performers agreement, you probably need to go over the heads of the local office and negotiate directly with ACTRA's head office. A brief google (= documentary!) on their website revealed the following clause (below) of a draft agreement with the NFB which seems the right direction for you to explore, especially as the alternative would seem to be abandoning your film entirely. Explain that you're not making a "for profit" concert film but focusing on the work with the children etc.
A2307 Waiver of Minimum fee for Documentary A Performer may voluntarily consent to waive his minimum fee for the inclusion of a performance, interview, or appearance in a Documentary Program, the subject of which is "the making of" the Production in which the Performer has been engaged.
Finally, I should point out that I am NOT A LAWYER and that you should make sure you have rock solid legal advice before proceeding with your film.
Fingers crossed that it all works out – keep us posted.