Ok, so although you are saving it with the intention of use by them, it's in your name.
I am no financial expert, a pure lay person. In your circumstances I would definitely have involved a professional at some point, even just for a consultation, even if I went on to manage everything myself. Getting those decisions right makes a big difference. I would definitely be checking about that 70k, as 10 years loss against inflation could turn it from a house deposit to half a house deposit, which is a significant loss in usefulness.
I think what I would do is invest most of the pension proceeds within an S&S ISA wrapper. It is then totally accessible should you need it, won't be subject to further tax (except inheritance tax, which is something to think about). Given that they have an (overly safe) 70k, you can take much bigger risks with this. I would view the whole thing as one portfolio, so this can be the risky bit of the money. I would go all shares, but in the form of a fund or etf. I have seen Vanguard highly recommended as a provider for low fees for both platform and their in house funds, which is a big consideration here. I don't use them myself but if you pop onto the FIRE starter thread people will tell you their experiences and recommendations. They will help you find one for your timeframe. This will help mitigate loss on the 70k. You will also be able to control when it is given to them, which sounds very important here. The major risk is inheritance tax, but presumably if that became an eventuality they would be inheriting from you anyway.
I would consider putting a small amount in a pension for them, like literally 40 quid a month or something. If one of them is impulsive and this might trip them up in later life, relatively small amounts invested now could grow enormously over their lifetime, meaning they have to save less later and meaning they are less likely to have a penniless old age. Its locked away so they can't touch it. My own father started a pension for me at 18, which has really reduced what I need to do now.
If you don't have a will, use some of this money to write one.
Finally one of the biggest investments you can make is in your own financial education. This is not expensive, but takes time. It will be useful for guiding them in the future, as well as for yourself. I found it hugely empowering to understand my own money. Understanding investment via funds and etfs, and the power of compounding, are hugely important. There are lots of podcasts out there. Again, the FIRE thread has recommendations on it.