PomPom (replying to your not understanding me)......
you said that your dd was looking on Rightmove at a property that would be £5K to rent, I just meant that you could explain the single maths to her. So, to replace your 2 bed with a 3 bed would cost you an extra £100000 to buy or an extra £300 pm in rent (obviously I've just plucked those figures out of thin air - you could do a quick shufty on line to get some actual figures), then show her what that would mean you'd lose (in terms of holidays, clothes you buy her, other things you do that she likes), for how many years, to be able to pay for the extra bedroom.
My dc occasionally used to stick their bottom lip out when I couldn't get to their assemblies or sports days etc due to being out at work. I said that I potentially could not WOTH, but they wouldn't be able to have a house with as many rooms as we did, nor to go on holiday, nor to go to music lesson, nor swimming lessons, nor go on cub camp, etc.,etc.,etc. Even much younger than your dc, they were able to see that me missing an assembly wasn't that bad after all. It's the same sort of thing "If I took on an extra mortgage / paid extra rent, then we would have to use all the money that pays for {insert the things she likes} to then put towards the new housing costs.
It's like giving them their own pocket money, they have to make choices, and then decide that whatever it is they wanted you to spend £££ on, wasn't so necessary when they are spending their own cash.