Day trip from Aviemore, Scotland

What I would do is to catch the 0832 daily LNER train towards London arrive 1115, and it's counterpart 1633 return, as they are the "best" trains of the day.

Is that too long a journey with the children?

The trains mentioned above are LNER ‘Azuma’ express trains to/from London. If you click this link and then find the trains at Aviemore - click on the desired train and you will see what it looks like and the number of coaches.

So, it is possible but I would not risk going today (Friday) as a very bad storm called ‘Amy’ is coming in off the Atlantic Ocean and this will result in services getting cancelled later on today - so you won't be able to get back.

Click this link to see the approaching storm:> https://www.windy.com/-Clouds-clouds?clouds,48.122,-18.062,5

UPDATE:> It now confirmed that the Perth to Inverness line (Aviemore) will close from 6pm on Friday 3 October due to the storm.
https://www.scotrail.co.uk/about-scotrail/news/scotrail-warns-travel-disruption-storm-amy-approaches

You didn't say you were going today, and anyway LNER are providing road replacement transport (unlike Scotrail) from Perth for the 1633 return today-

12:00 London Kings Cross to Inverness due 20:07 will be terminated at Perth. It will no longer call at Pitlochry, Kingussie, Aviemore and Inverness. This is due to a speed restriction because of high winds.

Further information
Customers travelling to Inverness and stations north of Perth will be provided with road transport. Please see station staff for further details Location for rail replacement coaches at the stations can be found here: www.lner.co.uk/travel-information/rail-replacement-stop-locations/ If you choose not to travel, a full refund of any unused tickets can be obtained from your original point of purchase. You may defer your travel up to and including Tuesday 7 October. We are sorry for the inconvenience caused to your journey today. If your journey has been delayed you may be entitled to some compensation. Find out more about our Delay Repay scheme at - www.lner.co.uk/delay

Scotrail are unable to provide road replacement due to the extent of the disruption.

And tomorrow morning (Saturday) there is a road replacement to Perth (where the train starts), again unlike Scotrail.

Thank you both. And no, I'm actually planning a trip for the spring. I think the kids will be okay because they are 9 and 7. We will have a baby but I'm assuming we can handle him as we do on a plane. This was so helpful.