The most important features in a keyboard to learn to play piano (in approximate order of importance):
- Full sized keys
- Complete keyboard of 88 keys
- Fully weighted keys
- Stand & bench adjustable to the proper heights for good ergonomics
- A place to put sheet music
- A sustain pedal
- Good sound quality
- Good touch response
- Other pedals (una corda, sostenuto)
There could be lots of debate about the order of importance. Personally, I learned piano for a few years before I was ever taught to use the sustain pedal, and particularly young children are often not able to reach the pedals, which is ok because learning to play without pedals is important.
Learning to sit properly at the piano is pretty critical, so making sure the height of whatever keyboard stand can be set appropriately is important, and even more important is the height adjustment of the piano bench. Generally you want the bench and stand to be adjustable so the thighs/knees can be raised up to just under the bottom of the keybed. If the player's elbows can be slightly above the keys, that's great, but that may not be possible for smaller children.
Having semi-weighted keys might be helpful for younger players, but at the same time playing the most realistic fully weighted keyboard will start the training on proper technique that much earlier.
Sound quality may seem like a trivial objective for a beginner instrument, but even young children can appreciate tone quality and will quickly learn to prefer hearing themselves making more beautiful sounds. One advantage of the piano as a first instrument is that the sound quality is only very slightly about player skill.
One thing you might consider avoiding is keyboards that have a lot of "features". I would look for an instrument that is focused on being a digital piano. You don't need a lot of different sounds or controls or anything like that, those are just gimmicks added on to try to sell instruments that don't have the best sound or keyboard feel, etc.
I suggest looking for an affordable "digital piano" - the kind that comes with a wooden stand and built-in speakers and is meant to be an affordable alternative to acoustic pianos. Many major musical instrument manufacturers make them, such as Yamaha.