Kamikaze420

on the other hand, it can be used to teach them about computer hardware when it needs an upgrade. Mom and Dad can upgrade their PCs, then maybe a Ryzen 5 4600 gets donated as well as a sweet graphics card.

Fortnite, Roblox and Minecraft are pretty dang low requirement. 
I still wouldn't go Athlon. Very few cores, and slow ones, Vega 3... It's barely gonna play the above titles.
As a dad, I am in a similar spot. What can I get away with saving? Will cheaper parts last a few years at least?
There are a few places I'd invest just a touch more to give this thing a few good years of life, so you're not putting more money into another system a year down the road.
PCPartPicker Part List Type Item Price CPU AMD Ryzen 5 3400G 3.7 GHz Quad-Core Processor $147.00 @ Amazon Motherboard MSI B450M PRO-VDH MAX Micro ATX AM4 Motherboard $79.98 @ Amazon Memory Patriot Viper 4 Blackout 8 GB (2 x 4 GB) DDR4-3200 Memory $41.99 @ Amazon Storage TCSunBow X3 240 GB 2.5" Solid State Drive $29.99 @ Amazon Case DIYPC Model X ATX Mid Tower Case $56.99 @ Newegg Power Supply Corsair CX (2017) 450 W 80+ Bronze Certified ATX Power Supply $49.99 @ Corsair   Prices include shipping, taxes, rebates, and discounts     Total $405.94   Generated by PCPartPicker 2020-01-27 21:59 EST-0500   3400G is 4c/8t APU with Vega 11. This will certainly work well for the intended use, and you could even slap a GPU in this build in a couple of years if her interests get more intensive without worrying about a CPU change. Included the 2x4GB RAM configuration, but honestly 2x8GB is barely any pricier, and again helps stabilize the system for several years of use. Here's a link for $10 more to double down on RAM. (I would.) 240GB SSD instead of a HDD will be enough for Windows (10GB) and the 3 games listed without any issue. This is the best way to start is with a solid boot drive and a bit of space. As she grows into the PC, a 1TB HDD can be had on a good sale for around $25 in the future. I chose this case as an inexpensive RGB option. It's got a white exterior, which I think would compliment pink lighting, and then you can utilize RBG software to turn the RGB fans and front elements to a fun pink. (And any RGB case could do the same if you or she doesn't like this option.) Skip the VS PSU. It's junk. This is the same price and much better quality.

Nvidia cards since 10th gen also work with Freesync now

I'll recommend this instead...
Significant increase in performance...
Windows key can be had for cheap from sites like kinguin, ebay and scdkeys.
 
PCPartPicker Part List
CPU: AMD Ryzen 5 3600 3.6 GHz 6-Core Processor  ($179.99 @ Amazon) 
Motherboard: MSI B450-A PRO MAX ATX AM4 Motherboard  ($99.99 @ Amazon) 
Memory: G.Skill Aegis 16 GB (2 x 8 GB) DDR4-3200 Memory  ($64.99 @ Newegg) 
Storage: Silicon Power A80 512 GB M.2-2280 NVME Solid State Drive  ($69.98 @ Amazon) 
Video Card: MSI Radeon RX 5700 XT 8 GB GAMING X Video Card  ($409.99 @ B&H) You can save a few bucks by going with a cheaper varient of this card. But this is one of the best 5700xt's out there.
Case: Cougar MX330 ATX Mid Tower Case  ($44.00 @ B&H) 
Power Supply: Corsair CX (2017) 550 W 80+ Bronze Certified ATX Power Supply  ($39.99 @ Newegg) 
Monitor: AOC C24G1 24.0" 1920x1080 144 Hz Monitor  ($144.99 @ Amazon) 
Total: $1053.92
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2020-01-27 01:33 EST-0500

Using Linux isn't that difficult. You will easily get the hang of it. 

I would suggest using Ubuntu or linux Mint as I found both very easy to use. I switched from Windows and Mac to Linux a few years ago and it wasn't that difficult. You don't really need to know how to use the terminal (which kinda scared me before I learned it) to use Ubuntu or Mint. Most of anything you need the terminal for is online anyway. I prefer Ubuntu now also and highly suggest trying it out.

Definitely pick mint if you want a good, and good looking experience. Its Fast, Easy, and it looks good