Get a black (for Charlotte, or brown for other type spider)outfit, sweats would great, but any black long sleeve shirt & pants would work. Get some black materialand make 4 sleeves, or get black shirts from Goodwill and cut the arms off. Get wire coat hangers, unbend them, stick one down each sleeve, stuf the sleeves with anything you have lying around. Bend the sleeves to desired shape to look like spider legs and sew them to the sides of the shirt. You could also stuff a throw pillow up under the back of your daughters undershirt, and tuck it into her pants to make a spider-ish hump. If she indeed Charlotte or other Black Widow, cut out a red hourglass or X shape and sew it to the front of her shirt. Use black (or brown, to match costume) face paint on exposed skin, being careful not to get it too close to the eyes.
The fishline idea others have mentioned is a good addition, I hadn't thought of.
Next we do angel wings... that is coming, you know.
M., mom of 7, grandma of 11, and therefore maker of many cheap costumes :)