This is one of the questions I and I'm sure many other photographers receive on a regular basis. My best advice is this: Wear whatever makes you feel comfortable! If you're wearing something that makes you feel any bit uneasy about your appearance, that uneasy feeling may show up in some of your photos. If you're wearing something that makes you feel comfortable, your images will look natural, relaxed, and confident. This is important for your kids, too! If you have an adorable hat, bowtie, headband, necklace, or any other accessory that you LOVE, think about whether or not your child will spend the entire session pulling at it. You don't want to be fighting your child to keep that cute little accessory on for the perfect photos. Your child will end up frustrated, as will you, and that's not what you want to remember!
With that said, I do have a few tips that seem to help people decide on the best comfortable outfit for the session. If there will be a number of people in the photos, you'll want to make sure that patterns are kept to a minimum. If one person is wearing a pattern, it's a good idea to stick to solids for others in the photos. If you stick with neutrals, you can really coordinate together quite nicely without putting too much time and effort into deciding on the perfect outfits. I know some people really like to coordinate, though, so coming up with a color scheme works to keep things coordinated without being matchy-matchy.
Because the bulk of my work is bump through baby, I have some stores in mind that work well for moms in these stages. For maternity and lifestyle newborn sessions, PinkBlush Maternity and Wren & Ivory have some great choices for all different styles! Both stores have plenty of nursing-friendly outfits, and some that can be worn both during pregnancy and after, too. Fresh 48 moms tend to go for a more relaxed look and I recommend looking for robes or pajamas from PinkBlush and Belabumbum. Really, you can go with something you already have, but if you want to treat yourself to something special, I'd go looking at those stores.