How can you make a very small bedroom that will only fit a single bed look professional and not childish?

You don't say if the room will be used as a bedroom on a regular basis or not... Assuming it's a child's room (full time or part-time, such as when you have custody) or former child's room in transition.

I converted a tiny 11ft by 11 ft bedroom into my home office. It also functions as a guest room. Since the space is not big and won't be used as a bedroom on a regular basis... I went with a sofa bed, to maximise floor space. I also used wall cabinets and wall shelves for the same reason.

My furniture-

Sofa bed (full size sleeper) from Jennifer Convertibles (pictured in link 1)
Desk- "Mikael" in birch, from Ikea (link 2)
Drawer unit- "Jonas" in birch, from Ikea (link 3). I got two of these. One does storage duty beside the desk. One is next to the sofa and is empty, for guests to store their things.
Wall Cabinet- ("Effektiv", in birch, from Ikea. I also added optional doors and mounted this above the desk. (link 4)
Office chair- "Jules" in birch, from Ikea. (link 5)

I also have a large closet, with sliding doors. I left the clothes rail in one third of the closet, for guest use. In the other two thirds, I put up shelves and drawers to organise my office things and store the bedding for the sofa bed..

In paint, I painted the walls with a dark beige/brown flat finish, then sponged on a white semi-gloss paint diluted with water to create a marble effect. The base colour I used is called "Pepper Spice" from Behr. After I got the white over it, it was much lighter and multi-tonal.

If you need a "real bed". Or just can't afford to replace it at this time. I would suggest adding bolsters and a slipcover to make it look more sofa like instead of bed like, if you will need a space for clients to sit. If you don't yet have a bed but will need one... You might consider a daybed which is slightly more narrow than a conventional single bed. The BedBath Store can help you outfit a bed with bolsters/slipcovers, and also outfit a daybed. (last link)