Nothing wrong with being a do-gooder, it's better than being a do-badder. I'd call the NSPCC as well, tbh. I'm not the most overprotective of mummies, but 15 minutes when your front door is open, is unusually lax. That child could have been killed - or bitten by a less well-behaved dog than yours!
And what a reaction, shouting at a 2 year old - wtf does she expect if she leaves her front door open and then doesn't go looking for her child for 15 minutes? Sorry, I know we don't know the whole ins and outs of it, but on the face of it, it looks dismal.