@WobblyLondoner I'm assuming stocks and shares but I'm aware that my DS is 14 so there isn't that long to invest for and I'm assuming we are in for a bumpy ride.
A junior S&S ISA will automatically convert to an adult one at 18, continue growing and be available for additional investments, so your comment above doesn’t make sense. Anything invested in the next 4 years will continue to grow until he decides to draw it out, hopefully not at 18.
Make sure the 9k is drip fed in monthly, to smooth out the peaks and troughs of stock market, and it should be fine.
I do think having money in Stocks and shares funds, where he actively has to sell them (rather than just withdrawing from a Cash ISA in a branch) is a good way of reducing the risk that he will draw it all out at 18.
My DC had S&S ISAs from early teens, that we paid into, and when they got an inheritance at 15 & 17, we just continued drip feeding it in, up to and after 18.
It helped them look at the inheritance as a long term pot of money, to be drip fed into the ISA and used for a house eventually. We also talked to them at length when they got the money and since, about how fortunate they were and if spent frivolously, it wouldn’t be replaced. 14 is certainly not too young to discuss future house or education costs with him.
DS - almost 23 - has gone one step further and started to invest directly in company shares, as well as funds, and enjoys monitoring them. Whilst receiving a large sum at a young age can be dangerous, it can also help them develop financial acumen.
As a pp asked, how the money was left will dictate whether any can be in your names. In my DC case, the solicitor was executor/trustee. DS was 18 when the money was distributed, DD’s money came to DH and I, but only after the solicitor discussed that option in private with her and we signed legal docs to become the trustees until she was 18.