The time period of the swing is given by     T = 2π √ (L / g)
The natural or resonant frequency is    n = 1/2π √ (g / L)
      L = distance of the center of gravity of child from the pivot.
      g = acceleration due to gravity
           1        √9.81
So n1 =  --------------- *  ------- =   0.3217 times per second
         2 * 3.14    √2.40Â
           1        √9.81
So n2 =  --------------- *  ------- =   0.3301 times per second
         2 * 3.14    √2.28Â
          Â
So the increase in the resonant frequency is :Â 0.0084Â times per second
            = 0.008 / second