But thb it's not only kids. If you have free time it would be better to exercise your body- your mind gets enough exercise at work already...

concerning professional advantages: after spending more than two decades writing software I seriously doubt that a side project gets you professional advantages.

Imho more important are soft skills that are not in the focus of most technically savvy people. Being able do communicate well and being a team player are at least as important in most cases- not everybody builds a new google...

We tend to focus on technical aspects while most fail to see that this is not a purpose in itself (in most cases / companies) but the purpose is to generate business value.