Hello. Thank you for coming to JustAnswer with your question, and welcome.
It's not likely that your cat would have been able to get any live viruses or bacteria from the plunger. I'm assuming the plunger was dry, which would mean any organisms that may be once been on it are dead. Even if the plunger had just been used, the cat would most likely clean its paws as soon as they became damp. So, again, the cat could not transmit anything by playing with the children or touching anything else in the house.
I don't want to provide more for you to worry about, but the cat touching the toilet plunger is no worse than the cat using a litter box, or even burying it's "business" outside. Cats are very finicky about their feet, and will keep them fairly clean.
Hello, When you opt out an expert, they are no longer able to respond to you. Even if the plunger had had blood on it, the blood would have dried and HIV virus does not survive on dry surfaces. The cat would also have cleaned itself well so there is no danger whatsoever to you or your children.