I would offer different packages such as 3 appointments in 9 months (subjective) for a certain rate as incentive to keep seeing you along with a one time only rate which would be whatever you consider adequate for your time!
You may also suggest having them use Fitness Pal, which is interactive as a way to focus on weight loss if that’s the issue.
I assume that the neighbors are not necessarily planning to enlist your services for personal training. This means that this could be an opportunity for you to display your skills, and I would present it as a menu that also has a package deal.
For example, the body fat and measurements alone could have one price, together with an aerobic test (such as the Rockport walk test) another, and yet another rate for a complete fitness assessment with pictures and all. I would charge my hourly rate for the whole thing, 50 % of that for the measurements with fitness test and 25% for the measurements alone. I would price it to make it attractive to get the entire assessment because this is a good starting point for a discussion on how to achieve improvements, and it may turn into business for you.