No, not backtracking at all.
My children live in a village of a couple of thousand people. They go shopping, in the local towns which have populations of about 20-30,000 people. There is no public transport, so no means for them to get independently to a big city by themselves (until recently when my eldest passed her driving test), so they've never done it.
They've never been on a tube alone, hell they've barely been to London, so like the OP's children it is a big step for them.
Whilst I didn't tell them my detailed plan, as the hotel and theatre trip was their first night away from home together, without adults, and my youngest had just turned 15 (it was her 15th birthday present). Yes, we did make it as safe as possible.
So we drove them to X station instead of Y station, because X station arrives at a different part of London and so they could get a direct tube to Oxford Street, rather than having to change, it's still 1.5 miles, so a bit far to walk with all your overnight luggage.
Hotel was selected as it was walking distance from Oxford Street.
From the hotel, they could catch a different tube direct to Leicester Square. About another mile away.
At no point did I write a detailed plan down for them. They decided everything themselves.
Yes, you're right, there are thousands of 11 year olds who do cross the tubes safely every day. My kids don't. But they do know how to cross a field of cows safely, which ones you can walk past, which ones really must walk around, what to do if the cows charge, they've spent their childhoods swimming in the local rivers, and know how to do this safely. Including life saving. They understand tides and undercurrents. They camp out alone, and can do many things city kids can't do. Hell, living in the countryside, you'd be surprised at the number of adults who can't cross a field of cows safely! The 900 cow attacks each year shows us that.
So it is a different skill set. And like the OP's children, they want to spread their wings, which I think should be supported, but if you have children who are naive about such things, it does need to be taken in small steps. A couple of years back, my daughter had to get the bus from the local town to another large town. We put her on the bus, and she knew exactly where she had to get off, but ended up two miles away, because she didn't realise you needed to press the bell when you wanted to get off. Of course, she's only used school bus before, and you don't use a bell on a school bus.
So yes, going up to London when you've just turned 15, live in the countryside with limited experience of public transport, working out which tube to get, how to read a tube map etc is quite difficult when you're more used to reading the mood of cows!