Cut the fabric to that length. The fun fur will curl if torn and will be perfect spider arms. Leave extra yarn at the top so you can tie the top costume arm to your child's wrists. When they lift their arms, all the spider legs go up. The family theme can be Mother Goose characters that complement the spider.
Size varies with size of sweatshirt.
Tail - Sew oval of white and black fir together leaving 1 end open. Length of tail should be length of sweatshirt. Use clear thread or line to attach tail end to neck of shirt leaving a few inches between the neck and the tail so it sways when you walk. Decorate hat or headband with fur ears and white fur tuft in the middle. Use pink felt for the middle of the ears.