This depends on the age of the child, their stage in learning, their own enthusiasm, their concentration span, and the quality of the time spent at the piano. In the very early stages when the fundamentals are being absorbed, as long as there is an understanding of what is being learnt, then little and often is probably best. As things become more complex then a little more time is needed to reach a mental understanding of what is being learnt, and a correspondingly increasing amount of time is needed to physically absorb it into the fingers. I am of the mind that it is important to have days off built into a schedule so there are guilt free non-piano days, rather than schedule practice every day and then 'not get around to it', but at the same time I know for some students it works to have it so ingrained into their routine, like cleaning their teeth, that they come to the piano without question and just get down to their work. It works differently for different families so decide what is right for your and your child.