A soft cushion for sitting, kneeling, or resting your feet on is a hassock. You can also call a hassock a tuffet, like Little Miss Muffet's tuffet in the nursery rhyme.
One place you can find hassocks is in a church in which the congregation kneels during prayers. These hassocks are also known as kneelers. If your footrest has legs on it, it's a footstool, but if it doesn't, feel free to refer to it as a hassock. The oldest meaning of this word is "clump of grass." Experts guess that the "soft cushion" meaning of hassock came from its similarity to a soft clump of grass.