Ex and I get on fine, 4 years post divorce. He rents a house with 3 bedrooms and has done since we divorced.
He has been with his new partner for 2 years, kids like her and they all get on well.
He called last night to say that he and partner are moving in together and would be telling the DC today (was nice of him to call beforehand).
I presumed she would be moving in with him, but he said that he is moving into her house (2 bed, one of which is a box room). I asked how that was going to work with the DC, he said it will be fine we'll work something out. I asked how they were all going to fit in a double room and he then said, "no they are going in the box room" 
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DC have stayed there a few times and two of them sleep in a single bed (top and tail) and 1 of them sleeps on the floor in a sleeping bag.
He said I couldn't expect him and partner to take the box room. I left the conversation at this point and said well you'll have to discuss it with the DC. He thinks there will only be a problem if I "put it into their heads there is a problem"
AIBU to think that it's not OK? They sleep at his 2 nights a week and 2 weeks in the holidays. DD has just started her periods and should have a space of her own and a bed of her own.