My background is that I'm a music graduate, specialising today in producing recordings of historical keyboard instruments, but with a lot of playing and piano recordings behind me.
Thank you for asking this important question.
In my opinion, for home use, a really good digital piano from a reputable manufacturer is suitable. This means properly weighted keys, a good sustain and, of course, the full compass. There is absolutely no substitute for the subtlety and tone of a genuine acoustic piano, but the ability to practice at any hour, and experiment with any kind of composition or exercise without risking the irritation of other household members or neighbours, are good things. Also, the instrument will always sound good, and never need tuning.
Avoid gimmicks, such as any kind of automatic accompaniment. Your child can learn to provide their own.
Do ensure that any headphones used are not so loud as to damage hearing. Please regularly check this.
And insist on regular practice of scales, arpeggios and other technical exercises as a supplement to and pieces, so your child's hands and fingers learn instinctively, over time, where to go. Please also regularly check playing posture and freedom of movement: an adjustable stool is worthwhile.
I wish your child many happy years of learning and playing once the initial technical difficulties are overcome. It has been a great joy to me.