Thanks for your advice, but the girls would like to sleep together. I am now struggling between a twin-bed size sleepover tent and an inflatable slumber bed. (http://www.dmartstores.com/palabedte19.html)
(http://www.playhut.com/playhut/playhut/sub/sub7.php?p=12)
The twin bed can allow two girls sleeping together in our sitting room, but the problem is that it doesn't come with a mattress, which means they will be literally sleeping on the floor.
The inflatable bed may be a bit more comfortable. However it has its downside of not fitting in any of our bedrooms. That means the girls have to sleep separately.
It is hard to choose.