We have been doing the same as MissSueFlay for a number of years, investing in both a JISA and SIPP with HL.
Initially we moved their building society savings into the JISA (we have saved with HL for years) then also started a SIPP. They are now 18 and 20, and were lucky to be beneficiaries of an inheritance 3 yrs ago, so that is feeding their ISAs and we have increased SIPP contribution a little, will probably increase more at some point. There is no problem with parents still paying directly into SIPP post-18, by the way, unlike ISA which converts to adult ISA on their 18th and needs new direct debit in childs name, and new fund selection setting up. Bit of a pain, we just done it for DD, but once set up its fine.
I too have enjoyed learning to pick funds and invest, was a complete novice yrs ago. Some have not been as good as others, but overall the growth has been very good.
The trick is to drip feed money monthly, into both ISA and SIPP, and into more than one fund, to spread the risk.
As for them wasting the ISA money at 18, well I do think education is the key here, and getting them involved in investing. Mine were 14 and 17 when they heard of their inheritance, almost 6 figures. We controlled DDs until her recent 18th, but DS was 18 when final distribution was made.
We spoke to both at length from the day that they got the estate estimate, saying how fortunate they were, how it would be a good house deposit, how if they wasted it we wouldn't help them. They have both taken it on board and DS doesn't touch it, except to move between accounts each month to feed his ISA direct debit.
DD is just at the start of that task, we are currently setting up new accounts for her, but apart from asking to use a small amount towards a better car (we contributed x amount, she added some) with promise of repaying it from her PT retail job, she isn't touching it. She has finished college and having a gap year, so picking up loads of shifts and has already paid some of car money back, so no problem there.
They have both been keen to get PT work, which I think reflects their attitude to not wasting the inheritance. They could easily have not got work, and broken into it for funds, but didn't. I appreciate that not all 18yo are the same, but education and getting them involved I think is the key. Show them house prices, work out deposits, ask them how long it would take to replace the ISA money if they fritter it away.