Once you choose a professional photographer, your photography period will be 100% for you and your family. You don't have to dress casually up like a plain white t-shirt and jeans.
You have to wear what makes you and your family look adorable. Your cameraman will make sure that their images capture each of the qualities unique to your family.
You can buy classy outfits for your family portrait session from https://www.thekindredstudio.ca/.
Anyhow, if you're still not sure what to wear with your family, simple, plain, and classic outfits are best. So make sure that the focus stays on your face and your vision stays forever.
Try to find common ground when choosing clothes for your entire family, for e.g. by using colors in the same tone which will help you connect the individuals in your image as a whole without having to dress the same.
Barefoot is always preferred, especially for young children, because at some point in the session they are bound to take off their shoes. Just bring some clothes for each child instead (and sometimes they get it wrong!).
It can also be a good idea to bring special pieces that may be sentimental for you or your child…hats, scarves, shawls can be great props and will add to your memories for years to come.