The best family sets are sold as a collection so every size uses the same print: baby, toddler, kids, adults and often a matching outfit for the dog. Check that every size you need is in stock before you order any of them.
Fabric matters for comfort. Cotton and cotton blends are breathable, flannel is warm and soft, and fleece is the coziest for cold houses.
Children's sleepwear has its own safety rules in the US: it must either be flame-resistant or fit snugly. That is why kids' pajamas in the same collection may be tighter than the adult version. Do not size up kids' snug-fit pajamas to make them loose. Wash new pajamas before photos so they fit and look their best.
Order early so you have time to exchange sizes before photo day, ideally by late November. Pair the pajamas with Christmas socks, and see ugly Christmas sweaters for party outfits.