I don't believe periodic charging would damage a smartphone, however it likely wouldn't charge it much either. A typical phone charger today outputs 1 Ampere of current at 5 Volts, and takes perhaps 2-4 hours to charge a phone. At 0.1 Ampere or a tenth of this current, it would take ten times as long to charge the phone. It would actually be longer since the phone is self-discharging during this time, and may not ever charge appreciably.
As long as the dynamo's output was rectified, filtered, and regulated to 5.0v, it should work for demonstration purposes. It must be regulated however, a phone will never expect 6V and could be damaged. It should also be "smooth" DC or direct current, meaning there is little noise or variations in it's output.
If the dynamo can be expected to produce less than 5V, then a "buck-boost", "boost-buck", SEPIC, Zeta, etc. converter/regulator can all output a fixed voltage (5v) for an input which goes below and above the target. I don't recommend inexpensive eBay items, but something like this "Step Up Step Down DC-DC Boost Buck Converter" can convert a varying input voltage to a relatively stable output voltage. Even so, I'd recommend having it output 6V, and use a second "buck" regulator (with better "line" and "load" regulation and less noise) to produce a clean 5.00v for the phone. Of course, the more pieces that are added, the more efficiency is lost. The best solution would be to buy (or build!) a custom converter designed around the particular dynamo being used. Questions about such would be best directed to electronics.stackexchange.com.
If you wanted to recharge the phone from a kitchen sink tap or garden hose quickly, consider a plumbing fitting which replaces the existing nozzle with an impeller under pressure from the water supply. The water pressure can do much more work than the free-flowing water mass and gravity, allowing the dynamo to spin faster and produce more current. Such topics as fluid pressure, work, fluid head etc, all fall under the category of Fluid Dynamics in physics.