OK, the scientist in me coming to the fore.
Try your own science experiment - get a packet of jelly crystals and made it up, double-strength. Don't use lo-cal, use the stuff with sugar in it, you want any bugs to GROW in this.
Example - our packet jelly crystals allegedly make up to 500 ml in volume. But try to unmould that, and it goes everywhere. To make a good stiff jelly tat you can unmould, you need to reduce the water content so a 500 ml jelly is only made up to about 350 ml.
Well, for this experiment, you make a 500 ml jelly to 250 ml - half the stated volume. To get the kid on-side, maybe make up more jelly and let them have some to eat later. Just label them carefully!
Find some shallow containers that you can put lids on - plastic round takeaway containers are great.
Step 1 - scald the plastic containers and lids by pouring boiling water over them. Tip it out, let them drain. DO NOT TOUCH THE INSIDE OF THE CONTAINER OR THE LID.
Step 2 - in a glass jug (also pre-scalded) make up the jelly with boiling water, to double-strength. Pour jelly mix into the plastic containers until they are 1 cm deep (about half an inch).
Step 3 - cover the containers with the clear lids then put tem to set in the fridge.
Now for the fun -
When you suspect your kid is germy, grab him and get him to put his germy hands on the surface of the jelly. Cover the container and leave it out (labelled) for a few days. You should see some spectacular cultures developing.
If the jelly hasset firmly enough, you should also be able to tip the container on its side and press it onto a doorknob, for axample, or anything protuberant. To swab a wall - a bit more difficult, but you could use a cotton bud - wipe the wall with a damp (but previously sterile) cotton swab.
If you want a control sample, get another swab out of the packet (being careful to not touch the end with your own fingers), moisten it under the tap and press it into the jelly. Again, let them incubate covered in a shady place out of the fridge.
For best incubation, you want them covered but in the shade, as if you were rising bread.
A suggestion - don't throw them out after the first 'reading' (about 3 days). Leave them for a week. Make sure difficult child sees the progress. A good recommendation also - document this for a science report for school. difficult child doesn't have to explain why it's being done, he can simply say, "I deliberately didn't wash my hands to see what we would grow." Take photos of the jelly containers at each stage, make sure you label them with an indelible felt pen so the pen shows up easily in photos. Don't just label the lid - lids can get accidentally swapped.
Now an important point after this experiment (if your child can understand the sophistication of this) - it is NOT healthy to completely eliminate germs from the environment, our immune systems need to be constantly inoculated at a low level, in order to stay healthy and not get out of control and over-reactive. Explain to the kids that it's like learning to cope with the rough and tumble of other kids in the school playground. Major bullying episodes are bad and need "antibiotics" or action taken, but the casual push and shove of kids sometimes stirring each other or teasing without meaning to be nasty - if we're not exposed to that, we never learn how to handle it ant not let it upset us. So we need to be exposed to a little of it, so we can learn to deal with it and not let it be a problem for us.
Germs are like this - really nasty ones, or lots of them, we have to get rid of somehow. But smaller amounts especially of less harmful germs, we need to be exposed to. And some germs, we really need in our environment. Some animals MUST have germs or they die (termites, for example; rabbits and rodents in general; a lot of herbivores).
But the reason we have toilet hygiene, is because THOSE germs are NOT good for us if we get them back into our tummies.
Hmm... maybe I should write this into a children's book?
Marg