B-2 visa visiting Canada

Hi,

My parents are visiting us here in USA. They entered US last Oct. and have had their visas entended for 6 months. They are holding valid I-94 Forms, but the original I-94 cards were not returned by USCIS (the card was sent for the visa extension.) Now, we are planning to visit Canada this summer, will they have any problem re-enter US from Canada? I was told as long as they are holding valid I-94 forms, they do not need new visas to enter US, is that true?

Your help is appreciated.

Joyce