Hey, I am also buying a Macbook (Air/Pro) for Unity development as well! To answer your question, yes, Unity3d does and should support Apple Silicon! Here's a link to their blog (if you scroll down, you'll see the screenshot with the description "Unity running on Apple Silicon":
https://blogs.unity3d.com/2020/09/1...s-now-available-and-we-welcome-your-feedback/
As for whether the Air or Pro is better for it, I initially thought the fanless (and potential thermal throttling) Air would reduce compile times. But after seeing all these benchmarks, it seems it only throttles up to 15% and only on heavy multi-core operations lasting longer than 8 minutes, such as video and image exporting. Coding compiling is mainly single-core operations, and compile times probably won't last for more than 3-5 minutes max. There was a video of someone doing code compiling on Xcode, and the two machines literally finished the same time.
Now I am reconsidering that I might go for the Air, because I can get more SSD space for the same price (and doesn't have the touch bar, which is a plus. Can't say your 9yr old wouldn't like playing with one though!). I ordered both a Macbook Pro 13" and a 8-core gpu Macbook Air, and will do compile time comparisons on Xcode and Unity. I was thinking about making a video on it and posting on YouTube since I haven't seen that yet! But right now, my guts tell me the Macbook Air may definitely be the better purchase (more demand, probably higher resale value too) and will probably return the Pro after I finish comparisons.
Hope this helps!