I TOTALLY hear what you are saying!! When your 2 year old knows the "Donalds!!" sign, that is not on me. Nor is it my fault when he/she knows all the words, to all the songs, to all the shows on Treehouse.
Having said that... reading between the lines here; sounds like mom felt a little threatened and that perhaps you were "telling" her what she should be doing? Feeling guilty about leaving her kids with someone else for a large part of the day? Still no cause to snap at you/take it out on you, though. Grrrrr.