Counting.  I really think that this is a good place to start.  Whether a student of any age learns music by reading standard notation or other method such as tablature, they (you) will need to know how to count.  These exercises can be fun because they are not hard, they don’t take a lot of time, and several family members can do these exercises together.  In fact, the latter would be ideal.  Anyone can learn this step before picking up any instrument.

Furthermore, once a student has grasped this basic part, they will be free to focus on the next steps.  You might think of it this way:

Learning to swim can be complicated by concerns about how and when to breathe without choking and avoiding sinking into the abyss.  After all which is important, right?   If a student could get around having to do everything at once (strokes, kicking, breathing, not sinking) and could focus just on the strokes (maybe using a snorkel, or flotation via parent etc.) then they can focus on breathing later.

Fortunately, counting music is way more comfortable and easier.

I’d recommend starting with the following simple exercises, and get this one mystery out of the way forever.  A young child can do this if parents and siblings will join in, so any age works. It’s not too late.

You do need to “count out loud to four.” 
Between each two vertical bars, count to four.  Just say “|  1   2   3   4  |  1  2  3  4  | 1 2 3 4 |” out loud.
Clap when you see a note.

That’s it!  Now try Lesson 1 together.
Allow about 15 minutes for about 5 days. 
Clap and then try other noise makers.  It should be fun.