I can offer some thoughts on that as well. Sweajnr is 3 yr 2 months and has done 5 return trips Calgary to LHR 😯 (The joys of being away from the rest of the family).
So LHR Terminal 5 – There is a soft play area for kids. It’s currently near A7 (I think). Right at one end of the terminal lookers left as you exit the security (but downstairs). It’s free and worth it to let them have a final run around. If you’re gates are B or C don’t underestimate how far / long it takes to get a toddler onto / off the connecting train. It’s a long way.
Assuming you’re in economy try to sit near the front as you’ll get served faster for food etc. I find they always try to serve me just as I’m trying to get sweajnr to sleep and so tell them I’ll collect it later which they don’t seem to mind. Don’t forget to pre-order the child meal. It’ll come early and hot but might have stuff they will eat. The “breakfast” you get served just before landing is rubbish so energy bars or similar is a good idea.
A couple of extra blankets makes all the difference in making them sleep. Have something handy for them to suck on during take-off / landing in case ears don’t equalize.
Just accept they’re going to watch a ton of digital media. In sweajnrs case 2 runs though of cars the movie and then he slept the rest of the flight. Trying to get them to sleep too early while the lights are on and food is being served is a bit of a hiding to nothing (in my case).
Depending on the size of the car seat it might be allowed on the plane. We tried this once but it didn’t work for us. We thought it might as he’d be comfortable in his “own” space but it was a tight fit. Getting him in / out was a pain and carrying the car seat though the terminals was a royal pain.
Enjoy Canada.
As I’m online and it’s lunch here any other Canada with toddler concerns I can help with 😀