I'm not blaming the kid at all. You're confusing blame with responsibility.

How, as specifically as possible, is 7 year old meant to ensure their own timeliness when they have almost no legal control over their actions?

Can't answer this as you're too focused on individual blame.

When I say blame, I'm referring to a moral judgment. If I'm blaming someone, I feel they clearly did something wrong and they should feel bad about it.

I'm not blaming a kid. I'm saying they're ultimately responsible. Kids and parents have to take responsibility for their tardies, even if they're not the ones at fault. As an adult, paying your bills is your responsibility. Even if some month you miss your rent payment for circumstances beyond your control, you're still responsible for making rent.

That's why zero-blame systems are a good way to design classroom instruction. You give the kid and parents a certain number of free absences and don't try to assign blame for every single incident.

Why would the teacher not be able to independently recognise the pattern of lateness and need a literal 7 year old to point it out?

That's exactly the kind of classroom I'm describing. You give a student a certain number of free tardies. For 90% of kids, they'll stay under the limit and not have a problem. No need to waste time litigating every tardy. But a few will consistently have more than the limit, and that's when you as a teacher can notice the pattern and start investigating.

And also, don't be too focused on the individual here. A 7 year old doesn't really have individual responsibility for anything. The parents are ultimately always responsible for the student arriving on time. Tardies are just recorded on the student's file, because they're the ones enrolled there. When a 7 year old gets poor marks or a disciplinary action, it's always a situation of shared responsibility between parents and children.