For iOS there’s AstroFlashTimer and not much else.
Don’t rely on the iOS clock time - it’s often off by seconds. If you want an accurate clock use Emerald Chronometer or their Observatory (iPads) these sync to NTP to milliseconds.
What I don’t know is whether the developer of AstroFlashTimer is aware the iOS clock is not accurate over the short term.
If nothing else you could use the i41CX emulator (it’s a programmable calculator with clock) and program this to issue a short audible beep (0.1s) every 5 seconds. Only snag for some will be the instruction set which is RPN and opaque to youngsters.
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