First of all son, if you find yourself asking your forty-two-year-old father whether the way you are intentionally mispronouncing a word is cool, this fact itself renders it uncool, even if it otherwise would have been cool prior to your solicitation of parental guidance. You don’t ask for permission to be cool – you just are. Cool is a quality you will into being, like a compassionate God or a sexy windbreaker. As much as it is contrary to my interests to say this, cool is almost never what your parents dictate, as parents are the very avatars of uncool.
Second, you’re five years old. You should be less concerned with trend setting speech and more concerned with keeping the sand from your playground out of my bed. When I was five, I was just happy to be learning the language at all, and never dreamed of bending my developing brain to the task of customizing that language to my whim in order to further my own cool factor. And who are you being cool for, anyway? The teacher who leads circle time? Just eat your string cheese and shut it with the cool already.
Third, why mispronounce a word to be cool? How about a colorful expression or a unique greeting? Why not add value with something new, instead of distorting the words we already have? Remember this summer when you questioned my authority by asking “Says who? The little man in the tin can?” That was brilliant. And cool. More of that.
With all that said – and with the caveat that I don’t know why you even want to customize a word you almost never have occasion to say – I suppose it’s fine to pronounce “booty” as “boo-tay”. At least as long as you’re not referring to pirate treasure. Or my ass, which as you know has a proper name, and also requires the prefix “Dr” before it out of respect for its postgraduate work.