Generally in a small room you won’t have much reverberation i.e. the echo would be less. Small room will have low volume so you should not have much of echo problem. For critical music listening We think small rooms might be problematic because of standing waves between the walls because the dimension, the distance between the walls is close to half wave length of low base frequencies. That is a technical thing so there are standing waves that happen in a room between two walls. So there is not enough length to complete the wave. So they form a standing wave and that frequency e.g. a room is 3 meters so the wave length which has a 6m wave length frequency that is about 50 Hz (not exactly). 50 Hz sound has a wave length of about 6m so that frequency will form a very strong standing wave between the two opposite walls. Hence 50Hz will be very high in the room. So when you listen to music and all that 50Hz will be little prominent and some other things will be low so those kinds of issues are there with base frequency in small rooms.
That frequency ranges for humans it’s not below 200, 150. At the most, for humans it ranges between 150-4000. And that is a regular talking frequency.
There are many factors that decide what room or what size of room will be best for children studying in the room. Even for music, lot depends on what kind of music system you are listening to, where the speakers is kept.