"9K a year, at 8% rate of growth of index funds on average, compounded over another 13 years"
OP, there are quite a few assumptions there. 8% is a very optimistic rate of return. Yes, there are funds which have done even better in the last few years, but who knows what the returns will be like over the next 13 years.
Also, stuff happens, you might not be able to afford to contribute that much every year for the next 13 years. I say this not to jinx you but because IMHO thinking about long-term investments requires thinking about how your circumstances might change.
The world is full of siblings who grew up in the same family, same education, same environment, same everything, yet one turned out to be a money-frittering t*, and the other turned out OK. Those who think it cannot happen to them are deluded.
Have you thought about stopping contributing after the ISA reaches a certain amount, then either investing in your name or setting up a discretionary trust fund? The latter is an expensive headache, really only worth it if the sums involved are large enough, but it does give you, not the child, total control, even after the child turns 18.
Investing in your name may be subject to tax, but you can still invest in an ISA (unless you max out your ISA allowance already) plus the first £12k or so of capital gains outside ISAs are not taxed.