When ours were very small and stayed over, DC used to bring the monitor and set it up for us. The 'moon/sun' clocks are a good idea as they knew they shouldn't get up when the moon is still showing.
You sound as if you have everything sorted out. The two of ours who stay most frequently start off in different rooms but quite often the little one would come and stand by the side of my bed in the middle of the night and stroke my face - making me jump out of my skin! Then I'd ask if she wanted to get into the double bed with her sister and she would settled down there happily.
(Older sister likes to have the double bed to herself, so they start off like that!).
Make sure they bring any cuddly toys etc which usually go to bed with them
Be prepared for 'just one more story please' - but I'm sure they will be fine.
Clocking Off - amazing BBC series from 2000!