What ultimately happens when a black hole evaporates due to Hawking radiation? [closed]

I hit a snag while trying to explain black holes to my son. I was explaining that black holes gradually lose mass through Hawking radiation. As the mass decreases, the Schwarzschild radius $r_s = \frac{2GM}{c^2}$ also decreases. My son asked what happens when the black hole becomes extremely small. In particular, is there a point at which the concept of an event horizon ceases to be meaningful? For a black hole with the Planck mass, $$ M_P \approx 2.18 \times 10^{-8},\mathrm{kg}.