We're pretty big HP fans and we've let DS2 and DS5 watch the first three. There are a few scary parts, but honestly DS5 was much more sensitive to the opening scenes of Cars than anything in the HP movies. We talk through the scary parts when/if necessary. Definitely no to HP4 though as it's far too scary.
At bedtime DS5 and I read HP. We're on book 3 now. So much more is in the book and the movies actually seem pretty tame in comparison. Had to put a stop to DS5 talking in parsletongue to DS2 because he was attempting to be scary. But other than that we haven't had any problems.
JB
(and DW and I have wondered aloud how much we are warping our children... so you're not the only one to ponder this question!)
