Trushna,
The positions for Mbed is as follows:-
- SW,
- South,
- West.
NW is not recommended as it is the direction of Vayu, which is very 'chanchal'. In case of your flat, despite the room in SW being small, it is better to go for that. NW for mbed is only if the rest of the bedrooms are in the eastern side. You can't put kids in SW bed as it may lead to clash between you two & not good stability either. The kids become more dominant in such case.
So, either accept the small bedroom or look for alternate flat.
Btw, tell me what is there in North-East & Brahmasthan?