The first thing I would recommend is to learn any legal liabilities you may have. Do not take the advice of anyone on the internet when it comes to legal matters.
That being said, the first thing I'd ask is whether you are operating as part of a company which may have company policies to follow. This sort of thing is certainly common, and they may already have an answer.
Failing that, the next thing you should do is acquire the Material Safety Data Sheet (MSDS) for your product. This datasheet should be made available by the company who produces the product. It should be the first place to go. I could not find a MSDS for the particular product you linked, but I did find one from the same company for their liquid sanitizer, which appears to use the same ingredients.
Now I am not in a position of authority on the precise risks with your "more fragile" people. I cannot say how they will react differently than a normal person. You will have to do your own research on that. However, to your question of whether a film might form, we can address that and provide some evidence that the product will not form the layer you are concerned with.
- The MSDS states that the product's solubility in water is "complete." This means that it will dissolve in water with substantial ease, and is unlikely to come out of solution afterwards.
- The MSDS lists hazardous polymerization as "Will not occur." A polymerization reaction would be the most likely way a product could dissolve into water and then become insoluble later.
- The MSDS lists hazardous decomposition as yielding nitrogen oxides and ammonia. You can do your own research on this, but these are not compounds that are known to deposit in solid form.
Between these, it is unlikely that this particular product will form a layer in your tank. I can't state it as an authority, but I myself do not see any indicators that would lead me to such a concern.
The instructions for use for this product will certainly call for some amount of flushing of the system after use. You may choose to flush it more times to provide an additional margin, but without being a medical doctor who is aware of the particular conditions of your "more fragile" people, I cannot say whether this will meet their needs.
Another MSDS for one of the active components in your sanitizer lists the LD50 as 240mg/kg (rat). For a small 50kg person, that suggests that a lethal does is on the order of 12g. Obviously that may or may not scale to your people in particular, but it does go to provide evidence regarding how dangerous or benign these compounds are.