I think you have no alternative but to support your two Dc in this way until they move forward in life. As students, they could never afford rent without an income. But surely, leading independent lives of their own, has to be the goal for both of them, starting with saving every penny right now.
You all seem happy with the set up for now, and very accepting of the unexpected turn of events, even though you never envisaged having the two toddlers living with you. So no problem to solve for now. You sound like a lovely mum/ grandmother.
But if you are going to have a big talk about your daughter’s wish to never move out, I wouldn’t go along with that. I would make sure she felt this was her home for now but I would also make clear that one day, she herself will want to move out, so that she has that firm expectation hanging in front of her, for her long distant future.
Also , I would want to make sure both dc fully understand that you now have a full house with no room for more babies or for gf/bf/partners. You have neither the room, the finances nor the desire to expand the household any further so you are passing the responsibility for that (birth control) firmly on to them. You need to say that very clearly because it seems to me, that your dc are rather comfortable and a little bit reluctant to face the big wide world independently, which just isn’t realistic.
I would be discussing difficult scenarios with them to plant the seeds so they know this won’t last for ever. Eg, you and your DH might wish or have a sudden need to down-size. One of you might become ill or disabled and unable to provide financially. You may decide to retire early, sell up and go travelling or move to Spain. In other words, their home security is not written in stone …anything could happen. The house is not theirs even though it’s their home for now. And they need to understand that and keep moving forward towards financial independence for themselves.
You have an instinct OP, that your DH, whilst never complaining, isn’t quite as comfortable as you, with facing the daily lack of quiet peace in his own home at the end of the working day. Make sure that you and your dc don’t ignore that.