They use a couple of the speed skating crash pads at my rink. The kids can go sit down when they've had enough. I think they also place a few toys on it. They have it out for every class, not just the first ones. They're heavy enough to not slide around but can easily be moved when they need to be. And they're the right height for the kids to climb on to.
I think it's quite good that it's something they have to get up onto, as it means when they're standing they're on the ice and it's only when sitting that they're not. There isn't that whole transition from rug to ice which I struggle enough with at competitions, so I suspect it's quite easy for a little one to forget what's where. They struggle enough with looking where they're going.