Edit: For the headless horseman costume they could have a long oversized shirt or tunic with their head sticking out of the middle. Supporting the ‘upper body’ at the top might be tricky… a pillow stuffing it would flop back and forth.
Maybe glue the shirt on a small cardboard box as if the box was wearing it, cut a hole through both in the centre for the face window, then lower the box onto their head like a helmet. The sides of the box will sit on their shoulders (you may need to cut curves into the edges for comfort and stability) and the length of the shirt will cover their body.
For arms you can stuff flesh coloured stockings and stitch straight lines for fingers, and just sew or glue them into the sleeves and to the shirt for ‘holding’ the kid’s head. Crumpled paper can be glued to the box through the neck hole and reddened with poster paint for the stump.
Alternatively they can just wear a pumpkin on their head
I’m a mouse… duh https://m.youtube.com/watch?v=PuNEzWXMiZo
Edit: For the headless horseman costume they could have a long oversized shirt or tunic with their head sticking out of the middle. Supporting the ‘upper body’ at the top might be tricky… a pillow stuffing it would flop back and forth.
Maybe glue the shirt on a small cardboard box as if the box was wearing it, cut a hole through both in the centre for the face window, then lower the box onto their head like a helmet. The sides of the box will sit on their shoulders (you may need to cut curves into the edges for comfort and stability) and the length of the shirt will cover their body.
For arms you can stuff flesh coloured stockings and stitch straight lines for fingers, and just sew or glue them into the sleeves and to the shirt for ‘holding’ the kid’s head. Crumpled paper can be glued to the box through the neck hole and reddened with poster paint for the stump.
Alternatively they can just wear a pumpkin on their head
That’s a great idea. Of course she’ll change her mind about what she wants to be half a dozen times between now and then!