RELAXED, FUN & INFORMAL!
Dress the kids simply, something that will co-ordinate with the room where you will hang your portrait. Bring along dressing up clothes. A scary pirate and a pretty fairy can make fun photographs.
Make your appointment when you know the kids are not tired or hungry. Bring along their favourite toys and hobbies, guitars, football etc.
NEVER tell your child to smile, their expressions will look unnatural.