I did a similar journey (including change at Crewe!) two weeks ago, with lots of trepidation. My DD (2.4) was suprisingly good, esp. when she started watching the DVD player we got specially for the trip. Great that you have that.
A warning: be prepared that (as TalesOTUnexp says above) they may NOT have given you the things that have been reserved ie table etc. We tried this too, and didn't get them. Hope you DO get them of course, but it might be helpful to imagine how to deal with things if not. My DD is just 2, but was booked across the aisle from me on one of the trains, next to another passenger...
Also: the air-con on our trains had broken, so it was excessively hot (no windows of course, so you could literally smell the air, full of sweat and not much oxygen...) - so just in case - have lots of water with you (a portable fan?) & be ready to strip down DS clothing.
Lastly: our DD reacted much more to the sound of the station announcer (maybe pitch/tone of voice) while on platforms (we had 3 changes of train) than she did to the intensely loud trains running through. The train noise she didn't like, but the station announcer voice made her burst into tears & become v distressed. So be aware of that, just in case your DS is the same.
I hope your journey goes really well - v impressed you've got assistance with bags etc - that's brilliant & should really help - changing trains with luggage & pram was the worst bit & quite stressful.
sorry I don't have any positive advice, but here is my info anyway. Good Luck!