The short answer is that you can only have one remote paired to a particular engine at any given time. This can be the remote that came with the engine, the 'universal' remote, or for 2017 and newer models it can be the bluetooth app on a smart device.
Both bluetooth and the more basic radio protocol used on earlier "second generation" LionChief engines work by being paired. Before a remote is turned on, the engine sits there yelling "Hey everyone! I'm engine number 12345, look at me!" When someone goes to program a universal remote (or bluetooth app) the remote listens for engines barking out their identity, then responds to the first engine it hears with a command something like " Hey engine 12345, I hear you. Shut up now, and listen to me. I'm remote number 43210". At this point the engine only listens to commands from remote 43210 and ignores anything else. The original remotes go through the same process, but the engine they will command is hard coded into them, so they do not have to listen to learn the name of an engine. So in the end, once any one remote is paired up to an engine, no other remote can control that engine.
It is just barely possible that a "first generation" LionChief engine would try to listen to commands form two remotes, but even if it did I suspect you would get erratic results as each remote tells the engine to move at a different speed. I don't have access to two of the same engine to test, but I wouldn't be surprised if these early engines simple pair up with the first remote they talk to, and ignore other remotes.
As an entirely separate subject, it IS possible, though a bit of work, to build a 'piggy-back' control system. You could hand the kids/visitors a remote that functions like a LC/+ remote, but that talks to a bridge computer instead of talking directly to the engine. Such remotes could also be used to control TMCC/Legacy engines if one wanted, giving visitors basic control of just a single engine. The control system could be programed with basic rules like speed limits and such, or it could rely on a user to oversee things and use their cab1/2 to take control if needed. The nuts and bolts is pretty complex, but the finished set-up could be pretty simple.
JGL
