To call an adult customer a fucking stupid lazy idiot would be grossly rude and completely unacceptable.
To call a 2 year old toddler, a baby still really, the same is beyond belief and I can't think of any excuse for it. Such a young child may not have explicitly understood the meaning of each word used but they would have been left in no doubt as to the aggressive tone used and would have been extremely upset and frightened, as the OP explains.
Frankly, if this woman honestly believes that 2 year old children are in full control of their bladder and/or that they go around weeing in shops to deliberately wind up shop assistants then she is the fucking stupid idiot.
I think MillieC should be congratulated for restraining herself and not slapping the bitch. I do know that really wouldn't solve anything but she must have been both distraught and furious that anyone could attack her toddler like that.
I wouldn't have a moment's guilt over demanding this creature gets sacked. You reap what you sow and all that. I don't give a toss about "bad days" in this particular scenario ..... Christ, we all have bad days. If everyone reacted like this you wouldn't be able to move 10 yards down the road without being sworn at.