There are many considerations when hiring a family photographer; here are a few (from a photographer's perspective):
Will you want the photos taken in your home (or outside it)? In a studio?
Or somewhere else entirely?
Will you want a photo book? Wall-ready framed pieces? Or just fine art prints to do with as you wish? It's a good idea to make sure the photographers you're considering can deliver these, and it's also a good idea to discuss upfront whether or not they plan on providing the final, full-resolution files to you.
What's your family like? Are there two or twenty of you?
Are we looking at a one-week timeline, or a one-month one?
Do you have any major events or milestones coming up that you would like to have the products in-hand for?
What products and services are "musts" and which could you do without?
If extensive travel is involved, what are you willing to reimburse?
Moral of the story: don't assume, ask questions, and be ready with answers. Good luck!