Today my DS age 16 months got quite badly bitten by his little buddy (18 months).
It was on his cheek, left a complete set of teeth marks and broke the skin a bit. A few hours later it is bruised and sore looking. 
The toddlers were being supervised by me and buddy's mum, initially I thought DS has just been clonked on the head or something. Anyway we both rushed over and picked up our dss.
The other mum says she feels really bad, has never bitten another child (but does bite her) and at 18m I don't think he really knew what he was doing?
DP is livid and says the other boy should have been really told off/ smacked because it's very bad to bite..