Converting child to adult?

I found a pattern for a child’s hat and I want to try and adapt the pattern to fit my head. Could I theoretically just use larger needles or would I have to take into account decreases and whatnot?

You can do this with larger size needles and heavier yarn but the proportions for an adult hat are going to be different so you’re going to have to knit longer before the starting the decreases. For the circumference, see what gauge you get in the brim and the pattern stitch and adjust the number of brim and body sts for your size from there.