Let's see some spiting of Muhammad to the face of minors in hadiths
Read Sahih al-Bukhari 77
Note that he was just a boy of five year old, and Muhammad spat on his face with the water from the bucket.
Now read this hadith Sahih al-Bukhari 188
Now read Sahih al-Bukhari 3910
Notice that the first thing that entered the child's stomach was his saliva.
When we read another hadith Sahih al-Bukhari 1502, we can see that this is a common practise amoung Muslims to have the saliva of Muhammd to enter on the stomach of children for blessings.
More hadiths can be seen about this in Sahih al-Bukhari 4151
Aslo read the Ibn Ishaq Sirat Rasul Allah page 117, here also we can see the cleanliness of handling food by Muhammad.
We can see lot's of spitting in Islam by Muhammad to invoke Allah's blessings or to cure any disease; But invoking Allah's blessing on food by spatting on it is the worse thing.
Read how Muhammad spat on the eye of Ali who was suffering from Ophthalmia, in this Sira, it never mentions wheather he was cured or not. But the hadiths that come later mentions he was cured like in Sahih al-Bukhari 3009. This happened during the Expedition of Khaybar
Read it from Ibn Ishaq's Sirat Rasul Allah - The Life of Muhammad Translated by A. Guillaume page 514
We can also see in some hadiths about Muhammad putting his saliva in the mouth of dead body. Read it in Sahih Muslim 2773a
