You don't mention whether or not you have custody of the kids- if you do and filing taxes with them as dependants your CCTB next year is going to be VERY nice... Nice enough that if you already have expenses dialled down you can throw it into an RESP for them and call it a day if you are so inclined. Also - if it's worrying you, you can double up the yearly max for any years that you didn't max out if you want to make up for lost time. (I am not explaining that perfectly so ask the folks at the ccra...I know for my older ones I do that as I started late )
Yes Canadians can invest in Vanguard, I do it.. Not hard. $50,000 is not anything but retirement$$ as a single parent at 39 years old to me, but I'm pretty risk adverse.