Can old small/invisble food stains/grease on furniture/food package etc. make child very sick? could i worry?

No. A little bit if dirt does not hurt children and may be good for them. Visit this site for information: https://www.fda.gov/vaccines-blood-biologics/consumers-biologics/asthma-hygiene-hypothesis#:~:text=According%20to%20the%20%E2%80%9Chygiene%20hypothesis,defense%20responses%20to%20infectious%20organisms. Wish you and your child good health!

There is a period between learning to walk and when they talk that most toddlers cram anything possible in their mouth to sample its taste or texture. Earlier they mouthed objects/furniture while teething. They build their brain experiences that way. I have recovered bug parts, foreign objects, food debris, etc. Luckily most survive the process. Other than trying to keep stuff clean, I ignore it.