Hm, I would make some kind of divider to give both kids some personal space, especially if they are of different age. Otherwise:
- Think of their hobbys, what do they enjoy? An easel and supplies for a little artist, a collection of something (stones, shells,...whatever you can think of. My sim kid collects everything owl related
) , an instrument, records/CDs and a stereo if your teen is into music - and so on.
- Lots of toys of course
- a desk to do their homework
- books for the little bookworm and a shelf to store them
- maybe a TV and seating? I think nowadays a lot of kids already own a TV and teens anyway
- for the girl a little vanity table - even if she's too young for make-up she might enjoy dressing up and putting on nice hair accessoires etc.