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 √2.40
1 √9.81
So n2 = --------------- * ------- = 0.3301 times per second
2 * 3.14 √2.28
So the increase in the resonant frequency is : 0.0084 times per second
= 0.008 / second