Just as pregnant women are "amazing and perfect," a normal immune system is also amazing and perfect. It actually operates best when challenged by the normal pathogens in our environment. Kids who get sick often in their first few years are the norm, and the good news is that their systems are able to mount quick and effective defenses against common infections by the time they enter school.
Having said that, there is evidence that the swine flu hits some people harder than the usual yearly strains, so I would want to take precautions against my kids (or myself) catching it, too.
I would ask people not to come over if they suspect they are sick UNTIL a couple of weeks after my children were vaccinated (which will give them the same immune response as having been ill with that virus). But be aware that if you don't get them vaccinated, they will stay susceptible to the virus if it hangs around for a year or two. You can't protect them forever from every possible source of infection, so immunization is usually a good idea.
As for the gel, that is rightfully your call. A good way to enforce an awkward rule for guests is to just take the responsibility for how you *feel* about it. Say something like, "I am *genuinely anxious* about my kids getting this season's flu, or passing illness on to others. So will you be so kind as to use this gel? I know it's inconvenient, but *I would feel more relaxed* during your visit."
Guests might argue with your science or your rules, but how can they argue with your *FEELINGS*?
BTW, there's lots of scientific evidence that people who consume or produce lots of vitamin D have stronger immune systems. Our skin makes vit.D when we spend time in the sun. During winter months, it's helpful to take large doses as supplements. A minimum of 400 I.U. daily is good, and using a natural vitamin D3 is best. (Some nutritionally-oriented doctors recommend up to 1000 I.U. daily for children, but it can be toxic over time to take too much, especially if you take D2, so I would seek medical advice on that.)