I assume its a 4 seater? If so then put the 7yr old behind you and her mum in the front seat on the right. That way she can reach her daughter easier than if her daughter is behind her. Also use a car booster seat (for the daughter!) so that she can see.
Most children don't get sick but choose a route where you can get back to an airfield quickly. Tell them to let you know ASAP if they feel queasy, then open every air vent and head for home. In 18 yrs I've only had 1 sick passenger and they held it together until we landed.
I always explain everything beforehand so that engine note changes etc don't produce panic. if you all have headsets/intercom then I always tell them what I'm going to do before I do it. Remind them to take a camera.
If it is rough or turbulent then consider leaving them behind. They won't enjoy 30 mins each way with bumps all the way and a 'wobbly' landing. Best they are disappointed than scared stiff.