In December, I travelled to Cancun with my wife and son (20 months old with paid seat), all three of us were offered free seat reservations on all flights. I did make the reservation over the phone with AC, so they made all of the reservations at once. Obviously that doesn't include preferred seats, and for the longer flight, paying the extra $50/seat for the preferred seats on a Rouge plane felt like fantastic value for the extra space. I realized that extra space is that much more appreciated when travelling with family than solo.
Of course, IRROPS can throw a wrench into your plans. This happened on one of the YOW-YYZ legs, where a last minute aircraft change from an E190 (2x2) to an A320 (3x3) meant we were stuck in an exit row with our son (not allowed) and the only remaining seats on the plane were middle seats (because of how they re-distribute seated passengers when going 2x2 -> 3x3). AC reservations was no help, and we solved this one by standing by for an earlier flight with more empty seats.
On the way back, we self-inflicted the same situation by standing by for an earlier flight (YYZ-YOW) that didn't have seats together. My wife and son found seats together at the back, and I was at the front. Once the seat belt sign was off, I moved myself to an empty seat closer.
Moral of the story: Don't expect AC to do anything to help you sit together if you haven't already. Even if you have, things happen...