I can relate to this too. When I bring our two out into the lounge floor space, they usually run to the side of the chair and cwtch by the radiator. This went on for a while at one point.
Usually, whoever leads, then the other will follow, even if it's Dotty (10 months), Maisy (4/5 years) will immediately follow very quickly behind her.
By doing similar to what Wiebke suggested, this helped a lot. So, I now put some scattered newspaper on the floor and/or in the areas that they stop, together with a cardboard hideaway and some hay and they seem to feel more reassured.
Often though, they will just 'sit' and 'stare' and I even say to them, "Well, fancy that, I've given you all of this floor space and more and all you decided to do is sit in a corner." The only response I get is a sniff in the air and little noises LOL!
Don't worry too much, I'm sure you're little ones will venture out a little further, as the time goes on. Ooh, plus another thing we've done as a family, is lay on the floor (not all together mind lol)
on our side and we've found that they usually come over and investigate and sit in the space where you bend your elbow. Worth a try!