If the built-in user switching is indeed broken in this iteration of Plasma — and mind you that Plasma 6.2 is just around the corner, probably arriving in the Stable branch in little over a week — then there is obviously no way to make it work now.
However, there is a workaround, i.e. have your children log into a tty, and have their login automatically start a new session on that tty. It’ll require some scripting trickery in their ~/.bashrc or ~/.zshrc, but it’s doable.