London Hotel booking problem-help needed

I will travel to London from US next week. I booked a hotel online. When I called to confirm they told me it is a twin room. I asked them if they can put a third roll-in bed in the room because we have three people(my child is traveling with us). They said no. So my question is what is a twin room? Is it really small? What's your experience and suggestion?

Thanks a lot.