What happens in the middle of a cat? That is like a koan. Seriously, I think as long as the child is dressed in white and has some gesture towards tail, ears and whiskers, no-one will quibble. You could make fur fabric paws if you felt that way inclined.
I got some metres of cheapo white marabou from a haberdashery - sewed it round the neck, sleeves & hem of a white t-shirt, round the ends of a pair of white kneelength legging things (both t-shirt & leggings from H&M bargain section), with a doubled-over length of marabou attached to seat of said leggings. White cat face-mask (made of foam, 99p) from Claire's Accessories - they always have them.
I made this about two years ago and it gets worn, and worn, and lent to keen dressing-up visiting friend, and worn, and ...
PS I can't do much more than sew on buttons, & for attaching the marabou all I had to do is make little wide-spaced catching stitches. Took me about an hour all in.