I personally think it is fine. Yes I was walking to and from school myself at age 5 and upwards in the 70's/80's. In fact I did it age 4 a few times. People will talk about there being less risk then but was there? I crossed 2 main roads and as for strangers, there were just as many crazy strangers then, we just didn't hear about it as much because we didn't have the fast paced constant news feeds that we have now.
Thing was, it wasn't lazy parenting or being reckless. We had road safety and stranger-danger drummed into us from the earliest possible moment. Crossing roads was a MAJOR daily topic in early years school, plus many parents then also took time to practice what was being taught on road safety. There was a daily consistent approach to actively teaching road safety. The two main roads I had to cross to get to school - they had lollypop ladies - but we were also all taught how to navigate crossing those roads WITHOUT such patrols in case we ever needed to be able to cross the road without help/supervision.
We, as kids, were all very engaged in it and as a result, we wouldn't have DREAMED of attempting to be reckless crossing a road or not simply walking to/from school the way we had been shown. If a stranger approached any of us, all hell would have broken loose. On top of which we could all recite our names, ages, DOB, address, telephone number and parents names off by heart in case we got lost and/or had to ask for help - and we were all taught who to ask for help and how to do it depending on where we were and what the situation was.
The culture that was created was such that, while kids could be and were often shits to each other, when it came to roads, cars, strangers and generally being safe when out and about without parents, the kids did look out for each other and knew what to do in the event of an problem.
The OP here is actually the OPPOSITE of lazy; she has spent time teaching her children and gradually building their skills in independently navigating a simple walk to school.
Teaching/building independence isn't about simply allowing kids to go off and be reckless. It's allowing them to grow and actively preparing them so that if something DOES happen, they are better equipped to deal with it - particularly if a parent is NOT present. It is all very well criticising parents who encourage independence, but if something happens and there are NO parents around, which kids do you think are going to handle it better?! Which kids are more likely to come up with a sensible plan to get help or until help arrives?
You cannot protect your kids against everything that happens in the world and you cannot prevent every accident or complication in life. What you CAN do is weigh up risks, take a sensible approach and TEACH kids how to deal with things - gradually building up so they become resilient and able to think through situations/problems.
My parents were quite strict in many ways - but one area they excelled in and one thing I am forever grateful for is that they taught us how to be independent and how to navigate things. It terrifies me how many kids these days are overprotected and who are growing up without the skills or mindset to deal with even daily situations never mind actual challenges.
A simple walk to school as the OP describes should not be difficult for 8 and 6 y/o siblings walking together and is, in my view, a reasonable and age-appropriate form of independence that will build self-confidence and sensibility around such a task. The OP has worked with her kids to get them to where they can do this - she's not just deciding one day to chuck them out the door and let them crack on. The approach here is perfectly sensible.