Question: I am doing my first photo shoot of a birthday party for a friend. I wanted to know which lens would be the better option to use for this. 
I have a Canon 50mm f/1.8 prime lens as well as the standard 18-55mm IS kit lens and a 55-250mm IS tele-zoom lens. 
So far, just getting to know my camera I have mostly played around with the kit lens. 
Now I'm not sure which of the others would be a better option.

Answer: 
I would suggest 50 mm and 18-55. Below are the reasons

1. 50 mm is great for portraits.
2. if the party is not well lit, you will not have lot of difficulty with 50 mm, but 18-55 will have trouble in low light.
3. 50 mm will not be wide enough to cover medium to large groups. That is where the 18-55 is going to help.

Having said that, if you don't have a second camera, it is going to be a pain to keep switching lenses and you might miss the moment.
If I were you, I'd shoot most pictures with the 18-55 and then after all the action is over, shoot portraits with the 50mm
I'd go with the 50mm f/1.8. Assuming you are on a crop-sensor camera a 50mm lens will be roughly equivalent of about 80mm. Combined with the wide aperture this will afford you the ability to shoot wide open or even at f/2 - 2.8 with a great depth of field and sharpness that the 18-55 just won't give you.