I think this question has been asked couple of times before here, but most logical explanation to this would be, muggleborns are not really under the watch from ministry of magic, are they? I mean it will know eventually that there is a kid in certain area who is capable of magic, and will send him/her the letter for magical education.
Another thing is, until they start the school, they are not really prohibited from performing magic outside of the school. When they start the first year, thats when they are told that, now that you have your magical education started, you are not allowed to do magic outside of school.