I'm not a big fan of abbreviated swearing. If you've left enough of the word there to be recognizable, then the word is still formed in the mind of the viewer - and it's still ugly. Try comic swearing, "&%^$!!!!", if you want to play it safe.
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Abbreviated swearing is not something I see as a Huge problem, but it should not usually be necessary or take place scratch projects. I agree with Paddle2See, swear words would be best to be bleeped out entirely. This is a kids website.
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Swap in a less offensive word, like 'darn' or 'oh man!'. It's a bit safer without using swears or censoring.
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