I'd be really interested to find out if anyone else can remember a TV show which was shown in the UK, I would say it was late 80s or early 90s. All I remember is that there was a pyramid shaped object in the room of a normal looking house, not huge, around the height of a child. The only scene I really remember after all these years is one where other children returned to find the side of the pyramid was open and one of the children had gone into the pyramid and was now unconscious (I think).
Based on what you describe this is almost certainly from an episode of Chocky's Challenge.
Broadcast on channel 3 in the UK in the 1980s. There were three series, Chocky, based on the John Wyndham Novel of the same name, Chocky's Children and then finally Chocky's Challenge
There is not a lot of information on line about this series but you can read a wikipedia article here https://en.wikipedia.org/wiki/Chocky_(TV_series)
and there are several episodes of Chocky's Challenge on youtube. The pyramid object you mention can be seen in Chocky's Challenge episode 6 at the 2 minute mark, the scene with the unconscious person inside the pyramid appears towards the end of the episode at 22minutes.