every $child in * satisfies $child[not(@my_attribute)]

will return true if no child has a @my_attribute that returns false. I.e., if the value were empty, but available, it would return false.

every $child in * satisfies not($child/@my_attribute)

will return true if no child has a sequence of nodes @my_attribute (alsways one or zero attribute nodes) that when normalized returns false. That is in this scenario effectively the same as above, I believe.

<child my_attribute="" />
<child my_attribute="" />
<child my_attribute="" />

will return false for the above test.
And the following:

<child attribute="" />
<child attribute="" />
<child attribute="" />

will return true for the above test.
And the following will return false again:

<child my_attribute="" />
<child attribute="" />
<child attribute="" />

Cheers,
-- Abel