While it's a shameless, self-plug - I'm finalizing a digital marketing course that's 2+ years in the making.  It's essentially everything I know, all laid out into a 12-week course. It literally teaches someone everything they need to know about digital marketing, from A to Z.  Since my background is sales, everything I teach is centered around being able to track results, and subsequently, ROI.  

There are loads of small business digital marketing courses but it really depends on what you're after. If you want to become a great marketer yourself you can't beat ANY of Cat Howell's courses. Facebook Ads That Convert (FATC) will make you one of the best FB marketers and advertisers in the business (and keep you up to date with everything that changes).

I keep up to date with FATC.  And I've just started her Incubator course to grow and scale my business, plus she has The Academy where she teaches how to build a multiple six-figure business (and beyond).

Digital Marketer also has some great courses although the style of them doesn't "speak" to me as much as Cat's do. :-)
 

There are so many out there and most are more than you need to know as a small business owner, unless your business is to be a digital marketer. 
 

Start with the beginners courses for the main pilars of digital marketing to have a good overview of each pillar. 
 

Moz for SEO

Hootsuite for Social Media

Google for Adwords

Other pillars to get a basic education on:

email marketing 

copywriting

networking

sales 

I have a daily show called BrandAPeel talking to small business owners and helping them with the basics so they can get a start on their marketing. 
 

Once you get some cash flow going you can hire people whose businesses are to provide companies like yours marketing campaigns for their specialized pillar. Giving you the time you need to do what you get paid to do. 
 

As Cate noted, choosing the right digital marketing course really depends on what you’re looking for. For example, do you want to gain a thorough understanding of a particular area of digital marketing? Or do you want to learn the basics so you can start rolling out different marketing strategies for your business?

That being said, some of the best digital marketing courses for small businesses today are:

• Australian Academy of Digital Marketing 

• Google Skillshop

• HubSpot Academy

• Udemy

• Coursera

• WordStream’s Growth Academy

If you’re looking for more bitesize learning, podcasts are a great option as you can learn on the go. Some of my personal favourites are:

• Marketing Over Coffee

• Social Media Marketing Podcast

• Marketing School Podcast

• The Hustle & Flowchart Podcast

i have found courses at coursera and udemy very useful.