I think it is important for everyone who learns music of any kind to own a metronome. Yes, we should be able to keep track in our heads; but a lot of times what I think is happening in my head rhythmically and the reality of the music don't match. And, yes, you can get an app on your phone for your students to use. But that means giving up your phone while they practice. Not always possible. Here are 3 suggestions:
Donner Mechanical Metronome DPM-1
Why? Well, your kiddos that come to my house love mine like this one. But it does take up room on the piano. Costs about $30.
Luvay Digital Metronome - Mini Portable, Multifunctional, Clip on, Beat Tempo
It’s cheap! But it is small, which could be a reason you wouldn’t want it. About $10.
Seiko SQ50-V Quartz Metronome
It stands up! About $26.