Surely the answer to this dilemma and to Osquitos, is to teach your DC the value of money and savings so that they dont blow all the money at 18? Talk to them about eventually buying a house, how much money they will need for a deposit. Tell them you have a nest egg built up, but if they blow it at 18 they are on there own, no begging for a deposit later!!
My DS is 18 so I know of lots of 18yos, I know none that have gone on cocaine benders! I think the majority are very sensible these days, esp given how much university and house deposits will cost. There are always exceptions I know, but with good conversations in teen years, you will hopefully not have the exception!!
We have been saving up child benefit etc for my DC since birth, we transferred it into Junior ISAs when they could have one. We use a Stocks and shares ISA through Hargreaves Lansdown. It think the min is 50/month, if you could stretch to that. We have paid a bit more, across 2-3 funds, but 50 paid into just 1 general UK or global fund would be Ok. Yes the share market is volatile, but by paying monthly, into a fund containing a variety of shares, you smooth out the peaks and troughs.
If/when you can afford more, add a second fund. My DC funds (and mine/DH) have grown more than any cash ISA would have done.
Although the money goes in their name/control at 18 (& I don't know of any account that wouldn't - they are adults at 18), I think they are less likely to withdraw it from a stock market ISA. Not as easy as going into a bank to close a cash ISA.