It depends on the cat.
In our case, we took in two very shy cats who had little experience with being a house pet (they were freshly retrained feral cats - but "feral" is really not descriptive of their character). The main focus for putting them in a single room was to prevent them from hiding away from us. The house is rather big with lots of nooks and crannies, and we feared that they'd never come out.
After a two weeks, we gave them the staircase (the adjacent space). They couldn't be bothered to check it until after another week.
When we eventually opened the house to them, it took them months to actually live in the other rooms and not just eat their food there.
So the question really depends on your cats. If they're very active and playful, give them enough space. Ideally enough space to run in a straight line, but not many houses have rooms to accommodate that so it might not be feasible.
Our cats were shy and calm. We gave them an above average bedroom and it was more than big enough for them.
Later, we also housed a younger cat who was (and still is) incessantly playful. We initially gave him the same starter room, but later moved him to a smaller room specifically to get him to stop running around nonstop.
None of the cats suffered from any of the other rooms. Especially if you let them out once in a while (we would shift the spaces by giving the younger cat the hallway for a few hours every day).
We will have toys, food, water, litter boxes, and a cat tower for them. There are also windows - and we will come in and spend time with them during this transition period.
As long as they have everything they need (food, water, sleeping spaces, hiding spaces, toys, preferably a window, visits from you), they should be fine. Seems like you've got everything covered.
If you've got some, cardboard boxes are always a nice addition to a room (for a cat).
a smaller bedroom that is used as an office. We will move out the computer and desk, allowing more room,
Unless you need it, you might want to keep the desk in there. It increases floor space by adding the desk's space to it, plus it gives some verticality to the room, which cats will generally like.
I don't want the 2, 3 year olds to be angry because of a too-small space while working to introduce the cats to each other.
Taking their feelings into account is commendable, but "angry" isn't really an expected outcome. Cats don't have societal expectations. As long as they have enough space to satisfy their needs (which mostly relates to playing), they'll be fine.
To prove the point, we'll be taking our cats with us once we move abroad. The cats will be in travel boxes for 12-15 days (quarantine). While it will be stressful for them, it's not harmful to them.
Comparatively, the room you're providing them will be massively more luxurious.
It's good that you care, but you should be all right. Unless that room is excessively cramped.
