My point is that forcing age-gates on anything provided via such formal systems incentivizes kids to go around those systems and install themselves an OS that doesn't do age-gating to evade it, not necessarily at school were they're unlikely to control the hardware, but at home.

Even before this, MS and Google have used their money to create a situation were very few of the formal systems for kids to access computers, such as schools, put anything other than their OSes in front of kids, so only kids who are naturally geeks/techies might have tried Linux out on their own - those kids would always end up trying Linux out because they're driven by curiosity and enjoyment from tinkering with Tech.

My point is for the other kids, the ones who wouldn't try out on their computing devices any OS other than the mainstream stuff that they've been taught about at school: with this law California might very well just have created a strong incentive for those kids to go around those formal systems and try Linux out on hardware they control, which not all will but certainly more will that they would if there wasn't a law in place to limit what they can do when using a mainstream OS - if there's one thing that is common in all societies and historical times is that teenagers naturally rebel against outside control and try and find ways around it, so limiting what they can do in the officially endorsed systems will push them towards alternatives systems which won't limit what they can do.

I think this might result in 3rd parties springing up as liability shields?

Sure, for a low price I'll be your "OS Provider"