Things you need to consider:
-Time of day
-Pictures you need to take: blowing out the candles, opening of presents, etc. (so think about important birthday moments and be ready for them.
-Who are the direct family members? What does the person paying you want pictures of
-Is the house well-lit or will it be dim? Depending on that many variables can change
Lenses? Something for low-light and portraits if you got it. Like a 50mm f/1.8. A wide angle is a good idea, telephoto if you got it, 18-55mm is always a default too. Be familiar with your flash, make sure you can use it.
The difference between taking photos on your own and being hired is that when you miss moments on the job, you can't go back and try again. So you gotta be ready to use your equipment. Fully charged batteries with a back up, a big empty SD card with a back up, and clean lenses. Possibly a tripod too, for formal, staged photos.