If its so important that you live in a world where you can let your child walk home from school at 8, move to a country that allows it.
Or be honest and tell your wife you can no longer afford child care. Because there has to be more to this than an obsession that you can only Foster independence by them walking home. Surely, you know lots of ways to do this, without expecting the school to change a rule for you and potentially be held responsible of something happens.
Don't your children play out? Do you have a good relationship with neighbours with kids? Here the kids are playing out at that age on the grass that the houses over look and we all keep out eye out. There's a shop about 200 yards away, that ds has been walking to since he was 7. Admittedly, I watched him out of the window alot at first. But he doesn't know that.
Even you taking them to the shop and letting them go in and buy things, while you wait outside, helps their independence. There's a lots of things you can do and work your way up
The obsession with walking from home, has to serve another purpose to you. Because you can achieve what you want to, without the school being party to it.
As for teaching your kids, to trust people. People shouldn't be trusted 100% a 100% of the time. That's a massive disservice to your child.
Especially, to your daughter who is at high risk of being sexually assaulted or abused at some point in her life. By someone she knows and/or trusts. Look up the stats.
A healthy amount of skepticism is not a bad thing and as a parent, you do your child a massive disservice but leading them to believe that's bad.
But there's something else at play here. Either GF or a parent that puts their wants ahead of their children or someone trying to hide something.
I feel for your wife, who would email the school about this before both parents even agree. Did you discuss it with her and she said no? And you ignored her? Or just decide you wanted to the schools position before you discussed it with her? You knew she would say no, so was planning on using the school to support your stance? Try and strong arm her into it?