Tips for shooting an event/Party?

[Ambrosia76]

I will be photographing my Granddaughter's 2nd birthday and wondered if any one has any tips. This what I have to work with:

My Gear- Canon 60d,18-55 lens and 70-300 lens,on camera flash only,which I have a bounce card and diffuser for,Lens hood. I am thinking I would like to mostly use the 70-300 so I can get back from the action and catch people just having fun and not just cheesing for the camera.

Lighting- I am pretty much screwed on this as the party is at a park at 3 in the afternoon. The park is sunny with patchy shade, with the exception of the picnic shelter which is overly dark. All I have is on camera flash,which I will try to use to combat some of the harsh light and will probably bounce or diffuse in the shelter. I have a 5 in one reflector set but not sure if I will get volunteers to hold them.

Ok so my biggest question is, I typically shoot in aperture priority or manual, but I am thinking with lots of little kids moving about I should shoot in shutter priority so I don't get motion blur. Am I correct in thinking that?

What about focal points? I usually use manual point selection then let the camera auto focus or I focus manually depending on the situation. But I am normally taking portraits of people I have posed or landscape shots. For this situation where people will be moving about and I am not posing them should I just go auto focus all the way and hope the camera gets it right?

Any other tips would be great.