Use 1/4 cup of chlorine bleach in 1 gallon of hot water. You'll need to use a rag (not paper) and that must be something you can 'throw away' because the strength of the bleach will shred it. Dip it and wring it out slightly and 'scrub' every part of the backpack well, then let it dry thoroughly before you put your baby into it. Do the part where the baby's mouth may 'bite or chew' extra well. DO NOT RINSE and don't use ANY SOAP or detergent. All you need is bleach, water and SCRUB.