Loch Ness is just another lake - there are closer places to appreciate the Highlands than going that far I believe. (There is no monster!)

KSOILES, your plans for Edinburgh are fine. I've not done the Hop On, Hop Off Bus there so can't comment. I find Edinburgh easily walkable. Be prepared for rhe hill that is the Royal. Mile but I've never found it a problem. Just organize your itinerary so you won't be making several trips from one end to the other. In fact the Royal Mile can be great gun for kids as they're usually pipers and street performers entertaining the tourists. You might want to have a look at the National Museum of Scotland. One of the best museums anywhere with a number of displays that your kids would enjoy.

I assume you are looking at a day trip for Loch Ness with a company like Rabbies. They are great and offer several excellent day trips. Some of their day trips include Loch Ness but visit other areas in the Highlands as well. You won't need to stay overnight if doing such a trip. Loch Ness may not be the prettiest or most interesting loch in Scotland but I image the kids may be intriqued with the idea of Nessie.

A Rabbies tour would be great - perhaps a bit long for the 6 yo. But you know your child and I don't. I would not take one all the way to Loch Ness -- that would be a lot of van time.

I'd look at a shorter option. Or even better IMO would be doinf either Stirling or St Andrews on your own. Stirling is quick by train, and St Andrews is train to Leuchars and then a local bus or taxi to St Andrews. St Andrews has a lot other than the golf -- Huge beach, wonderful castle ruin which is great for clambering, exploring, etc.

OR -- take a very short train ride to Linlithgow and visit the Palace.

https://www.historicenvironment.scot...ithgow-palace/

Any of those three day trips would be easier on the younger kids I think than a long ride up north with Rabbies.

Stirling would probably be my first choice -- but if the weather is nice, St Andrews would be terrific.