Hi!i know old pasta/rice can be dangerous,but what about a single pasta or some old grains of rice?if its on the floor unnoticed and a child eats it?or if it is squashed and stays on stuff?or dried pasta water somewhere? can they get very sick?

Toddlers are well known to have the habit of exploring tastes by eating stuff they find on the floor or anywhere else. This includes bugs, pet food, screws, twigs, etc. The potential for illness varies but many such ingestions are tolerated without illness.

Agree with Dr. Ferguson. What you described is unlikely to do any harm to the child. Wish you and you child good health!