The triangle scarf from McCall’s Costume pattern 4952 is shown as both a hip scarf and a head scarf. I think the hip scarf would be too fussy and uncomfortable on a 3+ year old—especially as I’m remembering that superhero cape from last year twisting around as she marched in her parade. On the other hand, I can’t see her keeping a scarf tied on her head, so I came up with a workaround.
I’m still looking for a child-sized ball cap, but you can see in the pictures below my idea is to tack it on in three or four places to a ball cap that can easily be taken off and put back on. I know she can do that, because she wears a ball cap for her local team all the time.