Why would your family "lose" priority boarding? You have the same entitlement to it as E50K, E75K and SE. Same goes for the priority security lineup (I think).
I have never been treated differently boarding with my family when I was E35K compared to E50K (early recognition).
Unfortunately, priority may be YMMV as the AC website does say priority boarding with family is for E50K up only. I have witnessed GA saying "no" to a family but the FF may have made the mistake of asking first instead of just doing it.
Per AC website: * Note: Altitude Super Elite 100K/Elite 75K/Elite 50K and Star Alliance Gold members may board with members of their immediate family (including spouse, common-law or same sex partner and dependent children).
http://www.aircanada.com/en/travelin.../boarding.html
I don't know about priority security, sorry, cant help there. I do recommend Nexus for you and your family, since my family all received Nexus and Global Entry, travelling has been much easier.