Seconding Johnjo's advice. Metal handcuffs of the generic type aren't pleasant at all, if using it to hold someone down while tickling. They're a great sensation for bondage scenarios, but with all the thrashing that comes with tickling...yeah, not a good thing.
Just as long as you're not going with the really cheapy stuff, like...I dunno, a pair under $15...most leather cuffs are quite comfy, great for use with tickling, and can be very strong depending on how you use them. The ropework you use will make all the difference, in this case...or maybe simply clip a lock between the rings on the cuffs, if you just want the generic "hands behind the back" thing. Go with the fur-lined cuffs...those are great. 🙂