Day Trip to Edinburgh from London

I have tangential questions about your proposal

1- how well do you sleep on trains

2- how many stops does it make on the way

3- just how well rested are you likely to be for a full day of touring

4- do you have plans for the 7th, (other than recuperation)

If you find my comments useless/irrelevant I will promptly remove this post. (I cannot imagine an overnight train ride with questionable sleep and then engaging in a 14 hr day of 'tourist stuff' being a pleasant experience)

I think a double overnighter is too tiring
I would suggest booking a late LNER or LUMO back to London, say 6pm or 7pm instead- which company doesn't in the end matter.
The last one leaves at about 8pm and gets back into Kings Cross at about 1am, but they are half hourly all day before that.
The sleeper that leaves at 9.30 is the Highland one. It stops at Edinburgh to divide into three but passengers can't leave there.
The Glasgow/Edinburgh one leaves a lot closer to midnight.
It's a good way to go, the train only stops a few times, basically non stop, except for crew changes.
By the time you've settled in, had a nightcap or a late meal you'll only get about 6 hours sleep maximum.
Some people can sleep on trains, some can't. If you can fine, if not rethink and maybe fly - early northbound, late southbound.
There are lounges at both Euston and Edinburgh, where you can freshen up and grab a coffee and eats.
In a day you can't possibly cover the whole of Edinburgh. Be happy with whatever you can and do visit that day, and prepare for a return for the rest. Of course Holyrood should be visited, but if you can't get there this time so be it. Don't feel driven by other people's tick lists, follow your own interests.
If you have the time an overnight in EDI would be far better to get 2 full days in, but if you only have the day it's doable.

And joe32f posed entirely relevant questions which I have tried to answer constructively.
He doesn't need to remove his post.
The UK sleepers aren't like those anywhere else in the world that stop at "every lamp post" during the night.

By the way the timings I gave above are the usual ones.
It did strike me afterwards that due to major work on the line north of Carlisle, on these dates the train still leaves from Euston but then diverts onto the East Coast main line.
The exact details of that diversion are irrelevant to the OP.
So the 9.30 departure is right, sorry. It leaves 2 hours early, and gets to Edinburgh 2 hours early to maintain it's normal arrival time at it's final destination of Glasgow.
That means that it is non-stop (for passengers) from London to Edinburgh.

Using the sleeper train is perfectly feasable and the only way to do Edinburgh as a day trip from London. There's plenty of other things to do in Edinburgh and although Holyrood House is interesting with a lot of history, it won't really matter if you don't see it. In fact with all you wznt to do, I don't think you would have time for it anyway!

I've always enjoyed Gladstone's Land, one of the oldest properties in Edinburgh and a fascinating glimpse of life of a wealthy merchant family 400 years ago.

Edinburgh is a beautiful city and deserves more than a rushed day trip. It needs at least 3 days.

It needs at least 3 days.

Not everyone has the luxury of being able to spend multiple days in every location. To me, a long day trip can be better than not seeing a place. Do what works for you.

I've done the Caledonian Sleeper from Edinburgh to London. I enjoyed it and slept like a baby for the journey.

Looking at the 6th of April the Sleeper has already sold out of their Caledonian Double rooms. So what they have available are either the Club or Classic. Benefit of Club is the en-suite room and breakfast is included. But as I recall other than that, the Club and Classic berths are the same size with twin bunks. I'm a solo traveler and while I was happy with my room and slept very well, they are not large and even for myself as a solo traveler it was tight quarters.

No one should delete comments, I like everyones input it has helped me planning many trips :)

To some of the questions above. We sleep just fine on trains, and can sleep anywhere :) We had considered Bath for a day trip but some of things there didn't really interest us or the kids who are 12 and 15. We are doing Windsor and Stonehenge on a day trip while in London also. My wife and I have done those all day excursions and while we didn't do everything in great detail we were glad we did it. We are in the mindset of we would rather see it then not see at all. We are all healthy, athletic and love being on the go trying to fit things in but also being realistic that we won't see everything in Edinburgh but that doesn't mean we can't go back. We just don't know when we will be back. Bummed that Holyrood won't be open and looks like the Cathedral is closed and not sure when it will be back open.

On the Caledonian Sleeper on the way back I was thinking of taking that on the way back to London as it would be more comfortable and we can sleep horizontal. And be rested, then getting back at 1am to Kings Cross and then going to our hotel which is near St. Pauls Cathedral. But I get if we get done with all the things we wanted to see by dinner, we don't have to kill 3 or so hours to wait for the Caledonian to depart. If we don't do the sleeper train on the way back. Is it easy to get tickets at the train station before departure even though it may be a little more on the day of on the LNER Train?

April 8th our plan was to see the British Museum in the Afternoon and have a lazy morning in London before we fly out back home on Sunday April 9th.

Claudia,
Had not considered those options. We go to the coast a lot in Oregon and California but wouldn't rule out the options you mentioned for Belfast. We feel like this kids would enjoy Edinburgh better due to the Harry Potter references/ sites (We are going to the Warner Bro Studios). But also seeing the stuff mentioned in my original post.

One thing I am considering is taking the sleeper train to Edinburg and then fly back to London in the evening. Easyjet has a 9:15pm flight back to London Gatwick

We were not impressed with the castle except perhaps the view and the one o'clock shooting off of the cannon but that is just one opinion. We really enjoyed Mary Kings Close and have done it on our three trips there. I do tend to go back to some things I like and fortunately on these trips I was with different family members so that was my excuse! The children might like Greyfriars Bobby churchyard, grave and the story. Even if it is not totally true we thoroughly enjoyed it. I totally agree to go.for.it. Who knows when you may have a chance with your children to.do.this again! Best wishes.and enjoy every minute.

Yes just buy tickets at the station if you travel by day- Super Off Peak Single £82.20 each.
I'm not seeing that St Giles Cathedral is closed generally, but that day is Good Friday, so when they issue their worship schedule for that day I would expect several services.
After 3pm, if they have been open earlier, they may not reopen until Easter Day for the solemnity of the occasion. Some Churches do that, or only open for services on the Friday.