Coledonian night train or LNER day train?

Maybe Coledonian train will be more comfortable than sitting on day train seat for 4.5 hours,

It’s the Caledonian. From “Caledonia”.

Only you know if 4.5 hours on a train seat is worse than 10+ hours on a train bed. But either are less than the 6+ hours on a transatlantic flight. I am a big fan of the Sleeper but I don’t travel with a small child.

Caledonian Sleeper is 7 hours from Edinburgh, not 10.
The new trains have had a lot of problems.
From 25 June the service will be run by the Scottish government. Who knows what may or may not change then.
I see the problem as being that you can't board until about 11pm. While you are allowed to stay in your cabins at Euston until 8am. the train (when it is running properly) often arrives soon after 6.30am, and there is a lot of noise around Euston. There is also an unscheduled stop for a crew change and breakfast to be loaded at Preston at around 3am, plus the Carlisle passenger stop at 1.45, and the shunting at Carstairs at just after midnight, to attach the Glasgow section.
With a child aged 7 and those kind of timings and happenings, I suspect you may be optimistic of "getting up fresh". Maybe a false economy, but only you can determine that.

I'm not a fan of night trains, but I've always just had a berth in a couchette, which is only a padded ledge. I'm sure a sleeper berth is more comfortable--though probably still considerably less so than a bed in even a relatively cheap hotel room.

The late-night departure is potentially a big problem here, I think. Train stations aren't necessarily the most comfortable places to hang out for multiple hours. If the departure were earlier, it would be easier to suggest that this experience might be interesting for your child. As it stands, I'm afraid it's just going to mean you'll be three sleepy people your first day in London.

I've taken a lot of train trips longer than 4-1/2 hours and haven't found them uncomfortable. I don't have children, though.

I took this train last year from Edinburgh. As mentioned, it doesn't leave until fairly late in the evening. So you would need to have somewhere to be and to wait until it does leave—with your luggage as well. I ended up bar hopping on Princes Street, but I was really just tired and biding my time after a long day in the city. Not a pleasant evening for me. Also, it really isn't that long of a train ride, so if you're thinking of having a meal (or two) on the train and a good night's sleep, well, there really isn't time for both. I got up when the train stopped, thinking it was a stop in between Edinburgh and London and I could have breakfast before moving on. But we were already in London. So I could have just as easily gotten off the train and had breakfast nearby. Yes, I saved a hotel night charge, but I didn't feel I got a good night's sleep. Fun experience, but I don't think I'd do it again. YMMV.

Here is a video of the LNER day train from London to Edinburgh.

Also consider going Edinburgh to York & staying 1 night in that historic city? That would break the journey and let you see something different. https://www.visityork.org

You can also travel on Lumo trains, by day, from Edinburgh to London Kings Cross. And every two hours on Avanti West Coast to London Euston.

I am a fan of night trains, but Edinburgh-London is a bit short for a night train. It takes a bit over 7 hours which is not enough for me at least. And it departs around midnight, something that might be a bit late for a 7 year old.

The day train should be easier, yes it is a train seat for 4.5 hours. But, book seats around a table and you can play games, or read or do other things to keep the child entertained. Not to mention lookin at the views. And a train seat is not as bad as a seat on a plane, even on the narrow trains in the UK the seats are much better than in the air. And you don't need to spend the entire trip in the seat, you can go to the café car and buy lunch e.g.

Thanks so much, for all helpful responses.
Looks like, timing is the problem with night train and I am agreeing to this.

@James : Current plan is to spend 2 days in London and then come back to Edinburgh. If it makes sense, we can break the journey or take alternate route, if it makes our trip nicer. When I say "nicer", our preferences are scenic and photographic places, be it in city or landscapes. York seems interesting, I just checked.

Earlier I had idea to get rental car, but then most posts are not in favor to drive between London and Edinburgh.

I will add that many years ago I rented a car just outside of London. We drove to the Cotswolds, Cornwall, and then up to Edinburgh. I don't recall driving between those places being any problem. But I do love a train trip.

Thanks for idea. Looks like a long drive - https://shorturl.at/eyFOV
I will check more on this.

I think that most people would agree that York would be the best place to have a stopover on the Edinburgh to London (Kings Cross) East Coast Mainline route. It has a historic centre including the magnificent Minster (Cathedral). Just on the west side of the station is the National Railway Museum - well worth seeing.

Note that another westerly route goes from London (Euston) to Edinburgh and the train company here is Avanti West Coast. It takes a little longer than the East Coast route (you only actually see the sea between Edinburgh & Newcastle). Also note that LUMO trains run non stop between Newcastle and London but most LNER trains stop at York.

It would be a long way to drive on some very busy highways and it does not sound like you have time to go wandering off to see anything en-route.

Here is a You Tube video of the West Coast route.

From York to Edinburgh you can also travel on Transpennine Express and on Cross Country trains. You have to change at Newcastle on TPE but you can make a virtue out of that, by using that as an excuse to spend a few hours looking around that fine city, or even make it an overnight stopover.
LNER are not the only game in town.
There's also no reason in principle why you couldn't go from Kings Cross to Leeds, take the scenic Settle-Carlisle train to Carlisle, then an Avanti or TPE train from Carlisle to Edinburgh (yes TPE run into Edinburgh up both East and West coasts). That would have to be booked in 3 parts- KC-Leeds, Leeds-Carlisle and Carlisle-EDI, but the 3 legs can be got for as little as £25, £12.50 and £11 per person. so is very affordable and very scenic north of Leeds.
Or even do that from York- a local train York to Leeds then as above. Book York to Leeds in advance and that is often less than £4.
Lots of options other than the blindingly obvious.

If you're doing at least two long train journeys like this it'll be worth your while getting a family and friends railcard for £30 which gives 34% off all travel for you and 60% off for the child.

Earlier I had idea to get rental car, but then most posts are not in
favor to drive between London and Edinburgh.

They are, and for a reason. If you're worried about your child not enjoying 4.5 hour in a train seat, 8 hours in a car seat doesn't sound like a better idea.

I have done the run between London and Edinburgh numerous times. Always LNER.

Last year, I decided to make stops along the way with a couple of nights in York and Newcastle. Still LNER.

I rode Avanti West Coast yesterday and it was fine. Transpenine Express changes management at the end of this month but I am still going to try to avoid them.

I will admit that I am willing to spend a little more for first class. There were seven passengers in my train car yesterday.

Very nice suggestion. Thanks a lot for all the help. I will check more deeply all suggestions.