I suspect I just mis-spelled it late at night to be honest - and I went and checked my old dictionaries just to be sure, but they don't even include infantilise, and use -i- to join other endings. Infanta is a Spanish word for "princess", so perhaps it makes sense that it's -i- not -a-! Hey-ho!

It was a really interesting discussion of feminising vs infantilising, though - I believe it was Nell Howell/Istia who pointed it out... All these things I wish I'd bookmarked at the time (though I suppose that wouldn't matter when people just delete their ljs anyway... *sighs sadly*)