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I'm happy that you have asked me to assist you and here is the limit chart based on a child starting at 60 pounds so I would not recommend much in a toddler's backpack:
Sorry, I pasted this and it dropped in the middle of what I typed!
I would not allow more than about 10% of the child's weight, so for a typical 3 year old 3-4 pounds.
Are you able to chat and did you see the chart OK?
Chat will time us out and kick us off if we are not posting something within 4 minutes.
Perhaps you've stepped away from your computer so I'll exit but stand by if you have questions.