I love watching budding artists!
The toothbrush technique works best with denser colors such as poster paints and acrylic; with oil, it has a tendency to smudge a bit (at least from how I tried it, but depending on the brand you might have different results). Although you can use a thinner, round-brush to draw the barnacles manually if required.
I'm looking forward to how this goes! Will you be using hue-mixing techniques for the grass?