You can use the receipt from CASSA toward your 2016 income tax. Note the following for 2016, 2017 and subsequent years:
The Federal Children’s Arts Amount Tax Credit was Reduced in 2016 and wil be Eliminated in 2017.
The 2016 Federal Budget reduced the 2016 Children's Arts Amount Tax Credit from $500 to $250, and will eliminate this credit for 2017 and later years.
Should you wish to write to your MP objecting to this reduction and elimination of this credit (and please feel free to do so; it’s been there for the benefit of your children), the list of current Members of Parliament is available here and the list by constituency is here. Letters to MPs don't require a stamp. A timely complaint or two may help reinstate this helpful credit.