Cattails set-up works because although her run is soil, it’s not very deep down to the slabs below. If you only have soil, chances are it’ll be a total mud bath in the winter. Also, you’re right, it will be difficult to secure the run against digging in soft soil.
On slabs, you can secure the run by putting slabs or bricks on top of the run skirt to hold it down - I’ve done this mainly because I worry that on very windy days it might take off (the clear tarpaulin cover acts a bit like a giant sail on very windy days.
I think you’ll find that even if they do end up coming out for an hour or so a day you’ll be unhappy with just a 2m run - if at all possible I would recommend an extra metre. I would also suggest 3 bantams in that set-up. Even though the house is easily big enough for more, your run wouldn’t be and crowding too many chooks into a small space is a recipe for disaster.
Probably the friendliest, most child friendly bantams are pekins - they’re also highly entertaining and easy to get hold of - however they aren’t best layers and they can be prone to going broody a lot. However these are generalisations as the pekin I’ve got at the moment lays on average every other day between March and September and hasn’t been broody at all yet at 2 years old.
Wyandotte bantams (I also have two of these) are generally known for being good layers - of my two one is OK (an egg every other day from March to September but with frequent serious broody spells) but the other hasn’t laid a single egg since passing the age of 5 and before that she only laid a few every year. Wyandotte’s are said to be friendly but although mine will eat from your hand they’ll screech blue murder if you try to touch them.
I also have a Plymouth Rock bantam who is an excellent layer and hasn’t yet been broody (she’s also 2) but she’s not overly friendly.
Then I have a Campine who is, if anything, overly friendly, and lays brilliantly - 3 eggs every 4 days, but she really hates being shut in the run.
And I have three Dutch bantams. They’ve become quite friendly, they lay well (an egg every other day or so) but small eggs, and seem to do ok shut in. None have been broody yet and they’re 2 years old too. They fly extremely well so you’d definitely have to clip their wings if letting them out.
I have had araucana bantams as well. They lay well, mine didn’t go broody. One was very calm but the other was completely mental!
So - long waffle but for you based on my experience I’d recommend pekins, Plymouth Rock bantams, Dutch bantams or araucana bantams. I would suggest you definitely don’t go for campines.
Im sure others will disagree / have differing experiences!