You’re going to be very crowded.
I would re-think the ‘own food & cooking’ - that means two time slots for the kitchen each evening, & frankly, they’ll very quickly end up with dibs on who gets it when, because they’ve got a small dc with additional needs who will need feeding. You’ll end up fitting in with them to sort out your own dinner. & then there’s the cleaning up - either waiting for it to be done, or trying to cook amidst someone else’s dirty washing up.
I would just dish up dinner for everyone & ask for a contribution to costs & that someone else did the washing up, personally, or would be happy to swap over so a couple of times a week they cooked.
But attempting to ‘timeshare’ kitchen facilities in a small house doesn’t work unless everyone is under 21 & mostly subsisting on Pot Noodles.
You also need to think about the living spaces: are they in their room every evening so you have your living room? Or are you all watching tv etc together? Where is dgc sleeping? Have your ds & dil got a space to retreat to?
How do you propose to monitor if they’re saving or not? They’re adults - you can’t check their piggy banks! Another source of potential conflict every time they order a pizza or one of them comes home wearing a new shirt…
It sounds challenging all round. I would definitely make it a fixed period arrangement - they move in for x length of time because they have calculated they can save y amount of money which will be enough for a deposit - if there isn’t a realistic forecast with an exit strategy, it’s not a plan.
Finally…statistically, there’s a probably better than evens chance they split up. Then ds is even less likely to be able to afford his own place, & you’ll be housing him permanently, with dgc part time.
Sorry to be the voice of doom, & I get you want to help out, but this should be the absolute last resort option - it’s really not going to be fun for anyone.
…oh & the dog is now going to 100% end up being YOUR dog, so you need to do some fast thinking about whether you’re up for that. If not, they will have to try to re-home it (& WTF were they thinking getting a puppy right now?!).