@SophieBxnks Sorry for the late reply (again); it's been a busy couple of days.
The build looks good, with all the components we talked about. It should be a great computer.
As for monitors, there are a couple of easy choices. With an Nvidia 2060 Super running Sims 4, any resolution below QHD (2560x1440) would be a waste, and even in QHD, the card will maintain something close to 144 Hz. It gets more complicated when you consider the monitor size. I think that anything over 27" would look a bit grainy unless it was 4k (3840x2160), but 4k monitors are more expensive, and the framerates are going to drop significantly, maybe by half compared to QHD (a very rough estimate). On the other hand, if your son is going to sit more than a couple of feet away from the screen, a larger QHD monitor would probably look fine.
The two best options, at least in my opinion, are a 25" or 27" QHD monitor with a 144 Hz refresh rate, or a larger 4k monitor with a 60 Hz refresh rate. Personally, I'd go with the first, because I like to be close to the screen, and even 27" is big. But here are a few different good options. The IPS versions have somewhat brighter colors than the VA panels, but the VAs are still high quality.
Samsung 27" VA panel, QHD, 144 Hz
Acer 27" IPS panel, QHD, 144 Hz (back ordered on Amazon, but maybe available elsewhere)
Asus 32" VA panel, QHD, 144 Hz (in case your son sits a few feet away from the monitor)
Acer 27" IPS panel, 4k
Acer 32" VA panel, 4k
Samsung 32" VA panel, 4k
These companies' monitors are all considered high quality and reliable. My highly subjective impression is that Acer laptops look a bit lighter, not washed out but just a slightly lighter set of colors. So it depends on personal preference. But monitors can be calibrated to the user's liking as well, so it's not crucial to get this exactly right. By the way, don't worry about the FreeSync designation—that's a technology that works with AMD graphics cards, and it takes some tinkering to get it to work with Nvidia cards, if it works at all. But it's also largely unnecessary, especially when running a game like Sims 4.
If I were buying, I'd prefer a QHD monitor with a higher refresh rate, but again, that's subjective. There's no bad answer here.