You have one bathroom and one kitchen between 4 adults and a ND DC. You’ll need a very strict timetable to make this work.
For example, if your DH usually showers at 7:30am before work everyday, then your son and DIL either need to shower the night before, or both be up and showered before 7:30am so your DH can get in, or recognise that they won’t be able to use the bathroom until your DH is finished (which may be too late if they work and commute and the school run is a long way).
Your DGS (& others) may need to brush his teeth in the downstairs loo in the morning, so you’ll need toothbrushes and paste in there, because you won’t be able to get in the bathroom to retrieve them.
If DGS and others are going to be using the bathroom in the evenings, you might want to put a time limit on that, depending on how loud the shower/extractor fan is. For example, if showering at 11pm is keeping the person/people in bed awake, then you may have to agree a “latest time” for showers at night.
Similarly with the kitchen. If you’re honestly planning on cooking two dinners every night, then you’ll need time slots. If you usually eat dinner at 7pm, do they need to have eaten and cleaned up before you start cooking at 6/6:30pm (possibly difficult, but a child-friendly time for them to eat), or do they eat after you’ve finished and have a late dinner (possibly fits in with working hours, but not a child friendly time).
You’re suggesting trying to live essentially separate lives (you & your DH Vs your DS, DIL & DGS) but every thing you do in the house will have an impact on the other “team”. It’s not undoable, but it is tricky and resentment will quickly kick in over “small” things, like the knock-on effect of someone being in the bathroom for 10mins more than the rota dictates, or a meal taking longer to cook which means other eating really late.
Throw in a puppy and a small child and that’s not a very relaxing way to live for two years.