It is a photograph OF a primary source. It is neither a primary source nor is it a secondary source (a commentary about the primary source). As a photo of a primary source, it falls within the realm of a primary source, but I would call it a copy of a primary source rather than a primary source in itself. In a post-holocaust world in a future time when the Bayeux Tapestry were destroyed and your photograph was a rare surviving image of that artifact, it would elevate to the realm of being a primary source.

Even a secondary source can become a primary source: Gibbon's work on the Roman empire was a secondary source at birth, but it can now also serve as a primary source as a means to understand how the Romans were viewed in a historic period.

If you want to get deep into archival theory, this would be a surrogate of a primary source, and the nature of what that means for studying history on the meta level is rather vigorously (and very nerdily) debated. I think you're totally fine glossing over the debate in a grade school setting though. :) If I were teaching in the archival setting (I have not taught that young, but I've worked with high schoolers) I'd refer to the tapestry as a primary source and not fuss about explaining to them that it's a picture of an object in a museum, I'd just assume the kids are bright enough to know what a photograph of an object is and is not.