It probably is a decent stepping stone but you have to be really careful to transition them and make sure they don't leave the king in check, move the king into check, or move another piece exposing the king to check.
With allowing the king to be taken, you have to allow all of those things, which may confuse them later.
I recently started teaching a library chess club. Many of the players are young kids who have never played before. When trying to explain checkmate, I realized... checkmate is really confusing! Of course it's second nature to experienced chess players, but when you step back and think about it, it's pretty weird. The goal is to get the king, but the game stops right before that actually happens!
So I was thinking maybe it's a good idea to play a simplified version of chess where the game simply ends with taking the king. Of course, this isn't quite "real chess," but if you think about it, there was probably a point in chess history where the game ended with taking the king, but checkmate was introduced as a courtesy or time-saver. Anyway, it's well-known that the best way to teach the rules is to start with simplified games. Anyone who's tried to teach chess knows that throwing the whole rulebook at a new player right away doesn't work very well.
Advantages of "take the king" chess:
So what do you think of "take the king chess" as a stepping stone for beginners?