When a child falls on a cement floor, his body comes to rest instantly. But $F \times \Delta t$ i.e. change in momentum is constant. As time of stopping $\Delta t$ decreases, due to which $F$ increases. So, child feel more pain.
When she falls on a soft muddy ground in the garden, the time of stopping increases and acceleration dcecreases and hence $F$ will decreases and she feels lesser pain.