Achild of mass 40.0 kg is in a roller coaster car that travels in a loop of radius 7.00 m. at point a the speed of the car is 10.0 m/s, and at point b, the speed is 10.5 m/s. assume the child is not holding on and does not wear a seat belt. (a) what is the force of the car seat on the child at point a? (b) what is the force of the car seat on the child at point b? (c) what minimum speed is required to keep the child in his seat at point a?

I attached the missing picture.
The force of seat acting on the child is a reaction the force of child pressing down on the seat. This is the third Newton's law. The force of a child pressing down the seat and the force of the seat pushing up on the child are the same.
There two forces acting on the child. The first one is the gravitational force and the second one is centrifugal force. In this example, the force of gravity is always pulling down, but centrifugal force always acts away from the center of circular motion.
Part A
For point A we have:

In this case, the forces are aligned, centrifugal is pointing up and gravitational is pulling down.

Part B
At the point, B situation is a bit more complicated. In this case force of gravity and centrifugal force are not aligned. We have to look at y components of this forces, y-axis, in this case, is just pointing upward.

Part C
The child will stay in place at point A when centrifugal force and force of gravity are in balance:

Lx bxh