It depends how small is acceptable.
If very small is OK you could use a low voltage DC power supply.
Or a soldering iron as heater.
At low voltage you can use eg Nichrome wire or tape wound around a heating tube.
If you can accept very hot near-boiling water and not steam it is "very easy" to get a controlled output by having water flow into a small mixing tank before exiting, heating the tank with immersion or clamp on heater and then monitoring tank temperature or exit temperature with a thermostat and controlling the heating energy.
For a toy or demo system the tank need not be closed or pressurised.
eg you can have a tank that water flows into, swirl mixes and then leaves via an overflow. Monitor tank temperature and adjust.
If you better define variables as above better suggestions can be made.