Being judgmental

I am currently on a paeds prac. Last week whilst on prac I was faced with the following situation, and from the very outset (ie reading the pt notes) I found myself being rather judgemental. From the notes and my supervisor, I was told that my 18 month old pt had been in and out of foster care from an early age. Although the babies mother had recently been found injecting herslef on a park bench at 2am in the morning WITH the child sitting next to her, she was fighting for permanent custody of her. Being fully aware of how the foster care system runs as I had a foster sister for many years whilst growing up, I was aware that the childs mother will probably be granted these rights and my supervisor confirmed my suspiscions. So, before I even met the childs parents I was thinking 'how could you do this to your child? They would be so much better off in foster care if youre on drugs etc' THEN the family walked in... my supervisor and i just looked at each other secretly going OH DEAR. The parents were swearing their pants off infront of their 18 month old, they both had their OWN pants hanging down by their knees, they stunk of smoke and alchol and had food bits dangling off them. Not to mention the fact that they were both morbidly obese but still managing to wear pretty much nothing. So at this time for one of the first times in my life I was lost for words. The little child ended up beinf one of the cutest little things I have ever met but simply delayed because she was ignored for the first 6 months of her life. As the session progressed, i found myself getting more and more annoyed because the parents didn;t even speak properly to their child and every single leyer of clothes the child had on ie 3 was covered in dirt and food and reaked of ETOH and cigarettes. Anyway, as much as i kept on telling myself to not let my frustration show, it totally did and my supervisor could tell (but luckily didn;t mind cause she was really annoyed too). Just wondering if anyone has effective stratergies to hide their unwanted feelings?