Are these sentences OK?
There is no pain that compares to the loss of a child.
There is no greater pain than loosing a child.
There is no greater pain than the loss of a child.

Question

Answer

With your second sentence, "There is no greater pain than loosing a child", you should replace "loosing" with "losing"... that is the correct spelling of the word that you are trying to use. Other than that, everything else looks good! :)

Answer

The first sentence and the last sentence are correct.

The second sentence should read as follows:
"There is no greater pain than LOSING a child."

"Loosing" is not a word. However, LOOSE and LOOSEN are. LOOSE is the opposite of tight. If something is too tight, you LOOSEN it.