The sentence is too direct and impolite. In English, you need to soften your language, especially when you are talking to customers. It is also better to use "do not" rather than the contraction "don't". I would also agree with putting the word "warning" in front of the sentence. I would therefore recommend the following:
"WARNING: For your safety please do not leave clothes on the heater"
Dime si quieres que explicatelo en castellano.