My yorkie tried to bite a child, please help?

i have a nine month old yorkie (and a female 2yr old (shes spayed, dont yell at me hahaha) shih tzu if it matters) and nemo (yorkie) IS my son, he is with me whenever im at home and NEVER leaves my side and he is very attatched and sleeps in my bed with me and whines (just for like 10 minutes) when i put him in his kennel/in the kitchen with baby gate... but anyways i was on vacation (my mom and sisters were still home) and they said he snapped at a (stranger, they just babysat for a neighbor that just moved in) 2yr. nemo was walking past the girl and she tried to 'feed' him some plastic toy food (my 2yr old niece lives with us and we nemo has only growled (BARELY!) at her maybe 3 times (because she stepped on his foot, pulled his tail etc)) and nemo lurched up and tried to bite her face!!!!! WHY?! every dog i have ever owned has never bit anyone! could it be that he was on edge because his 'mommy' (me) was on vacation? any tips on preventing this... ADVICE PLEASE!!!! thank you in advance (and btw, dont tell me its my fault, i didnt train the dog my mom did. she is the main caregiver)