Family sittings can be as fun or as serious as you like. They can be taken on location or in the studio, the choices are up to you. Once your session has been booked the next thing to think about is what to wear. This obviously depends on whether your session is inside or outside.
General guidelines would be to avoid big logos and branding on clothes which tends to date the images. Choose colours that are less likely to clash with other family members or choose matching colours.
For children it's always worth bringing spares of everything !