This is such a responsibility. You need to invest in your own financial education to handle this well. I recommend the Meaningful Money podcast. Take your time with the learning. As a competent amateur it took me maybe 5 years to really understand this, so don't worry about getting it all right. Just don't panic. Don't rush.
What is your relationship with your child like? Are you likely to be able to guide them? Are they likely to listen? Or do you need to safeguard it from potential poor decision making? These are important questions for you.
You also need to think about your goals for this money for your child. 70k could be divided into, e.g.
50k-60k ring fenced for house deposit (on the low side for now, but with investment growth and careful management it could be the right size by the time she is ready to buy, e.g. late 20s). 5-10k into pension. And then 5-10k to spend around 18 ish. On driving lessons/first car and maybe something like a painting or a piece of jewelry as a memory of her Dad to really enjoy and be tangible.
How to achieve this, what investment vehicles etc.
First, you REALLY SHOULD invest. People above saying premium bonds, or cash ISAs with interest rates, all that will happen is your money will have its purchasing power vastly eroded by inflation.
On the basis of the split I suggested above, here's what I would do:
With the company AJBell (I am suggesting AJBell so everything is in one company, as this makes it much easier for you to manage, get used to their app etc etc). I would open:
A Junior ISA (stocks and shares) - I will refer to this as a JISA
A Junior Self Invested Personal Pension - I will refer to this as a SIPP
A General Investment Account - I will refer to this as a GIA.
These are your wrapper accounts (literally think of them like your sweetie wrappers).
Inside them sit your sweeties/investments. I'll discuss these in a minute. First I want to talk some more about these wrapper accounts. Wrapper accounts basically determine how the money is treated for tax, there are rules around when you can withdraw it, and the government adds some free money into some accounts (yes really).
Before she is 18, each tax year she can put £9000 into her JISA and £2880 into her SIPP. The government will add another free 20% to her SIPP, taking it to £3600.
So 2022/2023 (before 5th April 2023) she should put £9000 into her JISA, £2880 into her SIPP, and £58120 into her GIA.
On 6th April 2023, she should move a further £9000 from her GIA to her JISA and £2880 to her SIPP. And repeat this each year until she is 18. For arguments sake, imagine you can do this for 4 years in a row. This means when she hits 18, you will have about 14,400 in her pension, 36000 in her JISA, and still have 22480 in her GIA. (In reality it will be more than this due to investment gains).
At 18, she gets control of all her money. There is nothing you can do about it. She can do what she wants with everything except what is in her pension. Though she is in control of her pension, she can't access it until at least 57. So at least £14400 is locked up and she can't blow it. At this point her JISA converts to a normal S&S ISA, with an increased contribution limit of 20k.
If she's listening to you, at this point she should open a Lifetime ISA too (I will refer to this as a LISA). A LISA has a £4k annual limit, to which the government adds a free 25%. So if you put in £4000, they will add £1000. You can use to either buy your first house, or access it at 60 (so as an alternative retirement savings account). This 4k is part of the 20k annual ISA contribution limit. So you can put 16k in her other ISA.
That first year, she should move 4k from her GIA into a LISA. And then another 16k into her ISA. And take what ever she wants from the GIA to pay for car/driving lessons/ a painting / a beautiful ring. Hopefully having a little to spend will, psychologically, stop her from blowing the rest. Then the following year, she she should move 4k from GIA to LISA, and everything else into the ISA. Then close the GIA.
Then every year after, drip feed 4k into the LISA from the ISA, to continue getting the free government money, until she then uses it towards her house deposit.
Caveat: I'm not quite sure how it works for annual limits in that transition year that she turns 18, but AJBell will be able to tell you.
In terms of investment inside these wrappers. I would look at Vanguard funds, as these are low cost. Probably the Vanguard Lifestrategy 80%. The same thing in each wrapper account. It is already diversified. The whole point is you don't have to think about anything else.
And finally, stick to the plan. If the markets drop IGNORE THEM, do not panic, do not sell, they will come back up. If you sell you will lock in the loss. You ONLY make a real loss or a gain when you sell, otherwise it's just a loss/gain on paper. Ride it out. They will rise again.
I hope this helps. With this plan you could help her buy a home, make sure she won't be in retirement poverty, and enable her to get a car/have a holiday/get a painting/driving lessons etc in early adulthood. It must be awful for both of you right now. I hope this helps a little. Take your time to understand.