Is fever and mucus presence a guarantee that diarrhea in toddler is due to bacterial infection and not viral infection?

No: Fever is a non-specific factor common to both viral & bacterial infections. Mucus is a more localized reaction to any germ but is not specific. Blood mixed with mucus/stool is suggestive but not conclusive for bacteria. There are always bacteria in the stool. A positive stool culture for known pathogens along with symptoms is good evidence for a bacterial infection as the cause.