Yes, but in your situation you went from 3 to 4 people.. very easy to do as when you are seated for 3 it is at a 4 person table. Not the case when you are 5 people, you will be seated at a table for 6.. so it is very likely that there would not be space....
I also went from 6 to 10, which meant adding another table entirely. Actually, we were at Sci-Fi so it was a whole additional car. They gave us 2 three-row cars right next to each other. That was in June when I swear there were a million people in WDW.
The other instance with 3 to 4 was over spring break a few years back. Yes, much easier, but again, it was packed so it's not like they had tons of tables. And our daughter was a year old so she was in a highchair so in some places that meant we were at a table for 2 with a highchair on the side (BIL wasn't there the whole trip, so before he came that was often the case).
And where we downsized, that was last week, the only week that they didn't offer the 4 for 3 deal, presumably since it was already pretty packed. Nothing like last June, but big lines at all the restaurants and downsizing was not an issue.
In the case where JuJu was downsizing from 9 to 7 and made to wait as a result, I would have just said "Forget it, the other 2 showed up, give us our table for 9". No way would I have waited that long without a complaint if the only reason was 2 members of my party got sick.
We have even been seated when we technically didn't have an ADR. In December, right before Christmas when it is also packed, we had the BC concierge book our meals and all were great except LTT for dinner right before
MVMCP. We got there and they had no record of our ADR. We had the concierge printout with us with the ADR # on it and they typed that in and came up with something totally random for another family, at another restaurant, in another month! We were seated in less than 10 minutes. And as we all know, that is THE place to eat during those parties so we thought we were out of luck. The staff actually apologized to us for not finding it and making us wait.
Same trip they also had my mom's room booked at YC, not BC with us, and even though the resort was "booked" they somehow got her a room next to us. In most cases, rather than see a family upset, WDW tries to right the situation whenever possible.
Sometimes the CMs who make ADRs do make mistakes, so it's pretty easy to say "Hmm, must be a typo cause there sure are 5 of us!" and I really jsut don't see anyone getting turned away for that. Yes, I see some people have had issues, but I am willing to bet those are the rare exceptions. Disney CMs generally are pretty willing to please and when they see what appears to be a family of 5 (as in 3 kids, not like she's bringing an extra adult), they would likely assume human error and seat them. I would be willing to bet over the course of the trip there would be at most 1 instance where they had trouble, but I bet they would ultimately be seated at all the meals.
We'll also be there free dining week, 8/23-8/29, and we just re-booked with a 40% off code we got. We had our choice of literally every resort, every view. We already booked in December with a bounceback for CBR pirate room, std view and were able to get the same exact room/view for less just this week. They had ASMo, Pop, POR, CR, Poly, BC, AKL, WL available in various views, and those were the only ones we asked about, so I am sure they had others. That leads me to believe it is not going to be super crowded. We also booked our ADRs at the 90 day mark and got every place we wanted (party of 8 in most places), with the exception of Le Cellier being a weird time. Still the day and meal we wanted though.
Anyway, my point is that in years of dining changes in WDW, if my only hesitation for not bringing my niece was meal issues, I would forget those issues and take her on the trip. The odds of something going wrong are really pretty slim and I wouldn't sweat about it.