Teaching children at a younger age will give them huge advantages in their futures. Not only that, but since children have brains like sponges, they will pick up languages more quickly and their accents will be more natural.
Ask for references from other parents. Find out what background the instructors have and what their specialties are. Check out ratings on ActivityHero and elsewhere on the web.
From online to in-person, prices will depend on the type of experiences being considered.