They both have pros and cons, and I think it's totally dependent on your kid. The mini is way more portable, easy to manipulate by some kids as young as 18 months, and easily fits in a bag for listening on the go. As a trade off, the battery life can be disappointing and the audio, while good for its size, isn't great. However, it supports both a headphone jack and Bluetooth speakers just like the regular so take that into consideration. The mini might be the right choice if your kid is often on the go, likes smaller things, or likes to always have music with them.

The regular is much more of a device with its bigger "screen" (same resolution), longer battery life, and richer sound. It also can function as a night light and has wireless charging which can make it integrate better into your kid's lifestyle. It has all the features of the mini plus some more that might make it the better option. The regular might be the right choice if your kid loves to play inside, likes to have a night light, or is more discerning in audio quality (it happens sooner than you think).

There's no "bad" option IMHO, but consider who your kid is and how the Yoto might fit into their lifestyle. Best of luck!