I'm looking for a new keyboard for my iMac so this thread would be of interest to me as well.
Although your child might not mind, keyboards can be very personal things. The feel of the keys, the key travel, how clicky the keys are are just some of things that might be important. You also need to think about how the keyboard will be powered. Wireless keyboards need batteries or to be charged every now and then. So the charge cable should be one that matches the ports on her MacBook. USB-C or USB? Otherwise adapters will work but that also drives up the cost.
Considering your criteria, I don't know of any that would be a perfect fit. Most backlit keyboards have a 10 key. Of the smaller keyboards:
Apple Magic Keyboard 2. No backlight. Wireless (built in rechargeable batteries). Appears to come with USB (not USB-C) cable. $100 at Apple.com.
Macally Mini USB C Keyboard. No backlight. USB-C. Wired (not wireless). Inexpensive at $30 on Amazon.
Logitech K380. No backlight. Wireless (runs on alkaline batteries). $40 at Logitech's web site. Round, not square keys. Comes in white and pink color options.
Backlit options abound on 10 key models at Amazon so I won't list them. Logitech has a new keyboard that works with Macs and iPads:
Logitech MX Keys 3. 10 key. Has backlight. Wireless (built in rechargeable batteries). USB-C. $100 at Logitech's web site.
I hope this helps.