To my knowledge, anyone CAN be picked to be a grand marshal, but you can't request to be picked. They choose families quite randomly except for very special reasons (for example, a terminally ill "wish child" can make special arrangements), so it's a matter of being in the right place at the right time with the right attitude. Unfortunately, birthdays are given no extra chance at being chosen, although it wouldn't hurt to have the child wearing a birthday pin while your entire family enjoyed the Disney afternoon!!