Some rules say 24 hours on medication (ie 3 doses) before returning to care but I have found that that doesn't make much of a difference since the child was ill before starting so germs have been spread and the one extra day doesn't prevent enough for the hassles. Depends on the cause of the infection on how contagious but in a family often only one has the ear infection. In a daycare group if it is spreading assume that they all came in contact with the same germ such as at a playgroup.

Sinus infection is an issue if there is a lot of nasal discharge - that is what is full of germs that gets spread around.

Generally I let them come if they can manage the day without needing too much extra attention. If I ask a parent to take a child to the clinic for confirmation and potentially antibiotics and they don't then they are sent home to deal with the child since that isn't fair to any of us.