Traditional slideway smoothing

A, the slideway is smooth to reduce friction, so A is wrong;

B, children in the process of sliding speed increases, but to overcome friction to do work, the total mechanical energy decreases, so B is wrong;

C, after the child slides down, because inertia can slide on the horizontal plane for a certain distance, it can stop, so C is correct;

D, in the process of falling, gravity does work on children, so D is wrong;

So choose C.