Two options, in increasing order of complexity:

https://au.yotoplay.com/yoto-mini

The parents would need to “burn” the required songs, obtained through entirely lawful and respectable methods, onto a blank Yoto Card. Each card supports up to 100 tracks or 500 MB. The player downloads the card contents and can then play them offline.

It is portable, screen-free, simple, and made for children. However, it is not an Echo Dot replacement. The child must choose from the music that the parents place on each card. OTOH, it DOES have inbuilt streaming and kid friendly podcasts.

(There's a larger version, with more capacity / battery life also - check the page yourself)

For actual Echo Dot-style functions, there is the M5Stack Core2:

https://shop.m5stack.com/products/m5stack-core2-esp32-iot-development-kit

It is a small ESP32 device with a touch screen, microphone, speaker, battery, Wi-Fi, and Bluetooth. The touch screen can provide large child-friendly controls for music, volume, playlists, and other functions.

The Core2 is development hardware, so it does not provide a complete assistant from the box. The father would need to install suitable software and connect it to a local server. However, once set up, it should be exactly what you want.

Eg: Server could run Home Assistant, Music Assistant, and Navidrome. Navidrome would hold the family music library. Music Assistant would connect that library to voice commands and playback.

With that setup, the child could ask for an artist, album, song, or playlist. The Core2 could also provide touch controls and spoken replies, so it's pretty much a 1:1. It just needs elbow grease. The main limits would be the setup work and the Core2 speaker (which may not match an Echo Dot for room-filling music, but could be pared with bluetooth speaker if really had to)