Translation:The dogs and the children are sleeping on the floor.
Why is "la planko" not in accusative here ("la plankon")? In another sentence in this unit, which included the preposition "sur", the object was in accusative case: "la infano versas akvon sur la plankon".
The accusative is used after a preposition only when there is movement towards the object.
La infano verŝas akvon sur la planko. - The child pours water on the floor. (the child is on the floor and pouring water)
La infano verŝas akvon sur la plankon. - The child pours water onto the floor. (the water is being poured onto the floor)
No, onto would not make sense in this sentence as there is no movement suggested. On indicates a location; onto indicates movement to a location.