Posters need to stop trying to find excuses to blame the airline.
From the flightradar trace and accounts elsewhere it looks like the crew spent a lot of time trying to resolve this, possibly during taxi out and almost certainly during the 10 minutes or so the aircraft was stationary at the holding point prior to the runway..
I'd be more than fairly certain to 100% confident that during that time the consequences of the child not being strapped in were made very clear to the family, probably more than once.
The basics are simple: if Canadian air law is similar to the rest of the world then child not seated and not restrained by a belt means it's not legal to fly.
Going back to gate to offload family and bags (security requirement) probably meant cancellation was inevitable (night curfew), and certainly the captain and probably the cabin crew would have been fully aware of that.
So IMO in terms of why the flight was cancelled well there's not much to debate -.it was due to the child's behaviour.
Why they behaved like that maybe is up for debate.