How might one obtain aircraft registration numbers for past flights (months ago)?
If you come up blank on the various open sources that have been suggested, which wouldn't surprise me, your best bet may be a personal letter to Air Canada and another as well to whatever headquarters address you can find for Jazz, explaining what you are attempting to do, and asking if they'd be willing to look up the aircraft ID for those flights for you.

For the right person, it's probably about 2 minutes of work total to look them up, although getting the request TO that person may take considerably longer.  Many airlines have customer relations departments whose job it is to address whatever issues people write in about, and a pleasant "we flew with you, flight was great, love you guys, silly us, forgot to get the tail #, baby's logbook, dedicated customers, please help, really love you guys, thanks millions..." letter may well get pointed to the right department and get you an answer.

You could also try calling and/or e-mailing; you're actually NOT looking for the standard front-line Reservations agent or "let me help you with your flight" type of person, but rather somebody in the Dispatch or Maintenance records section, as those are the people who have the big database of everything that the airline has ever done.  That said, it may be difficult to get past the usual barriers that airlines have in place to keep worried passengers in an airport from trying to talk to dispatchers about "why isn't my flight here yet???"  So I'd expect that the pleasant letter to a Customer Relations department may be your best bet.