A single school lesson lasts 45 minutes. There's rarely more than 1 lesson on specific subject per day. Do children not learn then? And much of those 45 minutes is lost on checking homework, making kids quiet, etc. I'd say 30 minutes of undivided attention is much more than a lesson at school.
Also, in six years this 0,5h cumulates to 1000 hours. It's not about teaching kids to be great programmers, as some say it takes 10000 hours to become great at anything. But spending 0,5h per day on different things gives children a choice in what they want to pursue later in life. And it gives them basic knowledge on numerous topics so they for instance are not cheated by a web developer the way I usually am by a car mechanic.