Nina. You could continue to let roots and leaves develop in water or you could place the plantlets on top of some damp potting mix in a small pot.
Either should work, but I would opt for the latter. However, you must keep the surface of the soil damp at all times for roots to develop. Spraying the surface of the soil every day or just before it starts to get dry may be a good way to water.
Use a peat-based, porous, soilless otting mix.