I was planning on making a pirate cake for my sons 5th Birthday. I wanted to stand a hook on the top of the cake but I wasn't sure if I should use fondant or gumpaste? I've never used gumpaste before but with my previous experiences with fondant, I have had a lot of trouble in the past getting it to harden. Any suggestions would be greatly appreciated!!

Unless you've got more than a week for the fondant to dry, and depending on how large it is so it would hold its shape, I'd use a mixture of 50% fondant and 50% gumpaste instead.

Or you can still use fondant, but just add some gumtex powder to it - it'll harden very fast that way.

agree, add vegetable gum or tylose to fondant or mix half and half (fondant /gum paste)
knead it and get the gum to absorb the more you knead it the firmer it will become. depending on the size it will dry.