Is it obligatory to make up for days of Ramadan if a child's saliva entered the mouth without knowing that this might break the fast, and is ignorance considered an excuse?

Your fast is valid, and you are not required to make up any part of it, as you are not certain about swallowing any of the child's saliva. Worship is not invalidated by doubt. Even if you had swallowed something, there would be no blame upon you due to your ignorance that such an act invalidates the fast.