If you are taking the medicine, because you legitimately need it,I recommend trying to take the adult medicine container, and making it feel more childish, rather then mucking about with dosages.
try taking fun duct tape and applying it over the front of the bottle, tying a ribbon around the neck of the bottle, you can even put the medicine into a fun child medicine pacifier, so that you can suckle it out.
HOWEVER, If you do not legitimately need the medicine, and you are taking children's medicine to enhance your little space, I have to warn you that mucking about with any Medicines is a BAD IDEA.
In order to do this with any certainty, you would need to know the various ingredients in the medicine, what they do, and what safe levels are for adults in comparison to children, yes you would need to take more, but how much more?
Granted, I know that of all the drugs you can be using to enhance your little space, children's medicine is most likely on the lesser end of the spectrum, that does not mean its any better of an idea though.