Hmmm. My guess is that children’s immune systems can do all sorts of things. There is such a thing as reactive arthritis, that I think can cause swelling. It’s found in joints that would seem unrelated to an illness. Like if you have inflammation in the digestive tract you might also get inflammation in the joints of hands and feet. Think about it this way, young children get a fever with a cold, adults don’t. They’re immune system can be very reactive. I guess also it could have been water retention. If the kidneys were strained? I’m just guessing. Food poising can kill children (God forbid) when adults typically can handle the same poisoning. You might remember a fast food hamburger E. coli outbreak at Jack in the Box that killed several children, but a few hundred people had been stricken with the infection. Most 24 hour flus and food poisonings that cause vomiting people can’t keep water down for at least the first 6 hours, that’s not uncommon. People torture themselves with trying to force water when they just throw it up within 30 minutes. Dehydration is a risk, but adults can gauge it for themselves. Children it’s more of an issue. In extreme cases the person needs an IV of fluid. What made them sick? If it’s camplobacter they might need antibiotics. I’m guessing it was more likely salmonella or E. coli Was it chicken? Beef? Salad and veggies? What did everyone eat who was sick? I’m not a doctor, just putting out there some ideas on the possible cause.