Timescales etc aside, I’m guessing the kids already knew your partner as you knew each other before you got together so that’s neither here nor there.
However, the fact here is that you have two children in a three bedroom house and that is small even by your own estimations. By moving in three more children you are all going to be on top of each other during times when all the DC are there and that is going to be a recipe for complaint, for bad relationships, for arguing etc and ultimately for the breakdown of the relationship if you both find it difficult.
With regards to your eleven year old being involved in the decision, children think in the heat of the moment and the reality here is that many children are excited in the beginning and then the relationships become more familliar and as things settle so those relationships change. Just because she loves the idea at the moment doesn’t mean she will once she’s a teenager and the six year olds are breaking her stuff because that’s what many six year olds do and there isn’t enough space for all of them in the tiny house you’re living in. Never take the good relationship for granted, because children grow up and feelings change.
You’re in a honeymoon phase of your relationship at the moment. That means that you want to be together all the time and moving in would make that possible in your minds. But it doesn’t take account of the rest of the circumstances.
From my own perspective, DP and I talked about moving in together after about a year. DC who was ten when we got together had frequently asked when dp would be moving in, had expressed a desire for him to move in in fact and I told him that there were circumstances which needed to change first to make that happen anyway so it wouldn’t be happening just yet. Fast forward five and a half years, and due to circumstances outside of our control it hasn’t been possible for DP to move to be with me, and as such we see each other on weekends only at the moment and will likely re-evaluate when DC leaves school.
In the beginning if you’d said it wouldn’t happen I would have been upset because I wanted to be with him, my DC and he got on extremely well, and I saw that as our future. But now it has been just me and DC during the week, and while he’s a teenager that has worked well for the both of us. Added to which, if he and DP ever clash I know that DP will be going back to his home town during the week and whatever it is will have blown over by the weekend, and we are all still happy and relationships have remained intact.
Looking back my personal view is that blending families rarely works out for the children because they don’t necessarily think the same as the parents and they don’t stay in the mindset they started out with when they’re forced to behave as siblings. And the more children there are the less likely it is to work IME.
This isn’t what you want to hear I know, but with that many children in the mix and that little amount of space I wouldn’t look to move in together at all for the foreseeable future. You can still have a relationship and still stay over etc, but making things that permanent sounds like a recipe for disaster. I would seriously wait until the children leave school. If the relationship is strong enough then it will go the distance.
Airbeds and sharing beds are fine for the odd sleepover at a young age but as your post suggests, you know this isn't a long term answer. It's not just finding them somewhere to sleep, but where will you keep all their clothes/toys/gadgets etc - if you don't have space for beds where will the extra wardrobes or drawers go?