Winogradsky column and hydrogen sulfide
Hydrogen sulfide can be toxic at high concentrations, however, the amount that your columns will make is likely very low, and will almost certainly degrade/oxidize into something safer before it hurts anyone (who isn't a small organism in the column!). It will noticeably smell bad at levels well below what is dangerous. So maybe just make sure that the columns are in a ventilated space.
Remember, what is happening in the column is basically just rotting organic matter, like leaving a bunch of food waste in a highly compacted, sealed environment. It is sort of intrinsically gross, but it isn't any more dangerous to your health than forgetting to take out the garbage for a month (actually, it's probably a lot less dangerous than that!).
For example, this guide says the following (after constructing columns with different inputs such as e.g. eggs):

You may have seen worms, snails, shrimp or other small organisms in the water, but probably not many (if any) in the bottle with the egg yolk, because hydrogen sulfide is toxic to most organisms!

So, depending on what you put in there, a very H2S-productive medium is likely to create enough H2S to kill tiny animals that are trapped inside the column. But apparently not enough to hurt large animals like you, as long as you don't huff the column.