Some things to think about before you get to your session:
If you're wondering what kind of clothing to wear for family pictures, it all depends on what kind of style you're going for. If you want to all match, I think simply black or white shirts is easiest and will look classic. I also think wearing all mismatched colors works as long as there isn't any sort of graphics, writing or pictures on the clothing. Honestly, when I plan my own family pictures I try to choose colors that match the decor in my house, because if I'm going to hang them in my living room I don't want them to clash! If you have more questions about wardrobe, feel free to ask!
One of the most important things I can suggest for you to do is to think about what exactly you want before you ever head out there. Are you expecting just family pictures, or do you want pictures of each child individually? Do you want some shots of you and your spouse? Do you want some of just the children as a group, have you seen a picture you'd like to imitate? Think about this and then let your photographer know what you expect so they can accommodate you. It is always disappointing to leave a shoot, get your pictures back and realize you expected certain things after the fact.
Are you up for anything? Or are you conservative? Are pictures hard for you to do, or do you have a spouse that doesn't really enjoy taking pictures? Do you have a child that is anxious or has a hard time sitting still? Does that child have a favorite game or something that makes them relax? Let your photographer know these little details and it will make your shoot go so much smoother. Or in other words, have good open dialogue with your photographer, good communication is the key.
If you are pregnant and want newborn shots, the best time to do it is two weeks and under. The younger the newborn, the sleepier they will be and it will be easier to get some of those really fun shots. So plan that ahead of time with your photographer so they can get there earlier than later.
Thats all I can think of for now, but most importantly: if you have a question let me know!