The velocity of a detonation wave in a suitable hydrogen-oxygen mix is probably not going to be higher than about 2km/s for a well mixed cloud with ideal fuel-oxideser proportions, and what you'll be dealing with is not going to be a well mixed cloud. Lets take that as a worst case figure.
Bits of debris are not likely to come firing out of the blast any faster than that. The blast is going to be approximately spherical, which means that most of the shrapnel will not be heading towards your viewpoint. It'll arrive as a cloud of irregularly shaped chunks of lightweight metal and composites.
This compares favourably with the speed that you might be expecting meteor strikes to have, which could come in a lot faster than that... up to 72km/s, in the extreme.
The minimum safe separation distance, then, is a function of how much energy your windows can absorb per unit area, and some model of the most dangerous bits of debris coming out of the explosion. That seems entirely too hard to work out, as far as I'm concerned, but with suitably over-engineered windows you could handwave almost any distance away.
does prudence say just never do that?
There are good reasons to be cautious here. You're flying a nuclear reactor strapped to a bomb in front of a viewing deck.
Personally, I'd consider being about 1km away from a debris cloud propagating at 2km/s to be an OK distance. Consider that you have half a second to activate emergency protective systems... a set of safety radars might be able to trigger an explosively-closed blast-shutter fairly promptly, throwing up a curtain of rock and metal that could be as over-engineered as you saw fit.
These ships fly themselves - to hit other structures they'd have to be quite off their final approach and their emergency systems would have to all fail. (I don't know that 'self-destruct' is the way to go in this case. Maybe...)
One possible scenario here is an uncontrolled descent onto the spaceport at de-orbit velocities, which is also going to be at about 2km/s, only there's going to be just one big lump of debris instead of an expanding cloud most of which will miss you.
The roof of the any of the buildings on the surface will probably be quite thick! You might consider that point defense systems (that after all, might be powerful fast tracking laser weapons that could present a serious threat to anything hundreds or even thousands of kilometres away) or range safety systems that would allow someone to remotely destroy a ship, problematic for other reasons (especially for the owners and any passengers!) but then you're in the territory of viewing areas being too risky to allow.
There's also an additional problem you may want to consider.
A bang which throws up debris at 1-2km/s in a vacuum is going to throw that stuff a long, long way. A lot of it is going up and out, and it could pose a serious threat to anything in orbit. That's a reason why you have landing pads by the way, instead of landing on unimproved regolith, because otherwise the debris kicked up by your landing burn could trash the person behind you in your original orbit...