Hi Lisa,
It's best to get the flash off camera, so I would recommend getting a light stand and a sandbag to weigh it down for safety reasons. These don't have to be hugely expensive but if you can stretch to a good quality stand, it's definitely an investment. Personally I wouldn't use the flash on it's own. Fired directly at the animals/people it will give harsh light and hard shadows, which will make the images look a bit amateurish. You do have the option of bouncing it off a ceiling or wall but, you have no control over the spill.
Ultimately it's going to depend upon the mood you are trying to create in the image but, as it's a happy event, I think the lighting should reflect this and be low contrast, with no ugly shadows on the backdrop or subject matter.
Perhaps the easiest and most versatile modifier is an umbrella. You can get ones that have a removable cover that convert from a reflective to a shoot-thru, give you different options for containing or spreading the light. The size is up to you but get something that is going to work for the environment you're in. I'd suggest at least 60cm, preferably closer to 90cm-100cm.
If you're using a Nikon (and the model has the feature) you can use the CLS system to fire the flash off camera. You don't mention what camera or flash type you have. Is it TTL or manual. Do you possess any radio trigger system?