According to Wookieepedia (entry title "Midi-chlorian"):
1) "The magnitude of the midi-chlorian count served as a measure of one's potential in the Force, though there were other inheritable characteristics that could influence Force ability as well. Indeed, though Force ability often meant a high midi-chlorian count, it was not always the case."
2) "Midi-chlorians could be detected through a blood test by measuring their concentration in a being's red blood cells, though such tests were not perfect and were prone to fault."
In other words, testing Palpatine as a child by no means would have guaranteed the Jedi Order of discovering his potential in the Force. And of course the Galaxy is too vast for every child to be tested. It's likely that only those children who demonstrate unusual skill that might suggest Force aptitude were tested, as was the case with Anakin.
I always take things from Wookiepedia with a grain of salt. The same entry states:
"During the reign of the Empire, midi-chlorian tests were performed routinely on Inner and Mid Rim worlds to root out Force-sensatives and Jedi in hiding. Such individuals were rarely heard from again. In response, an underground trade in drugs a blood treatments sprang up that could supposedly fool a test or lower one's count; however, they were largely ineffective."
According to this, the blood tests were so effective that Force Sensatives became desperate for a way to beat them.