How about an overnight train trip? They combine travel and hotel in one and there are two overnight trains in the UK (Paddington to Penzance and Euston to Scotland (Edin or Glasgow, or the highlands)). They don't run on Saturday evenings though. They are well suited for families with kids, though not particularly spacious. Note that the trains set off late in the evening which could either be delightfully exciting for your kids or a nightmare for you. You could stay a whole day somewhere and return on the overnight train the following night, or just get on the first train back after arrival, perhaps visiting some other places along the route if that's not too expensive.
Have a look at The Man In Seat 61's website which has information about train travel around the world (also ferries though you've said you're not keen on that): www.seat61.com/
With kids that young you could probably do quite a lof of pretending that you're on holiday without actually leaving the house. Sort of Advanced Staycations I suppose. There are a few examples of families inventing their own during lockdown, such as this Austrlian family that recreated a 15 hour flight to Munich in their living room, complete with boarding passes, security checks, microwaved TV dinners etc. On 'arrival' they changed to German cuisine and used virtual museum tours to visit various European sites of interest (Google Streetview is also fun for exploring a new city online) www.theguardian.com/australia-news/2020/apr/16/coronavirus-vacation-australian-family-recreate-15-hour-holiday-flight-in-living-room, another example here www.birminghammail.co.uk/news/uk-news/watch-family-whose-holiday-jeopardy-18133471 (just google 'family recreate holiday lockdown' or similar on Google for more).
Maybe your 6yo can recreate some famous paintings or landmarks for the Family Holiday photo album. Insist on using a few words from the relevant language (might as well learn something while doing this!).
As your alternative holiday plans gradually become a little more fluid you might go down the route of the frankly bonkers Experimental Travel which is where you do things like holiday at your nearest airport (or service station, Gloucester Services is really lovely), or even Ikea. My favourite is the Expedition to K2, where you get a large map of your chosen area, find whatever is in the grid reference for K2 and go and visit it, planning your route to take account of other items of local interest :)
www.latourex.org/latourex_en.html
You've said you're not keen on a ferry to go somewhere but have you thought about a short family-friendly cruise where you're ON a boat all the time and the journey IS the holiday? I am not sure what the current recommendations are (Covid) and how expensive things currently are, but it might reduce some of the travel-stress feeling because you're not constantly having to think about needing to repack and leave. Many cruises visit different cities so you can leave all your belongings on the ferry and explore at leisure. This is generally quite a bit more expensive than a Holiday Inn trip of course.
Good luck, and bonne vacances! :)
Jo