These are the questions you need to ask yourselves when making provision. People like junior ISAs because it provides them with a further £9,000 ISA allowance where growth and withdrawals are tax free. But the downside is they cannot get at themselves (legally!) and the children have full access at 18.
Trusts are an option but even if the trustees wish to retain monies in the trust, an of age beneficiary can sue for the money. Plus trusts require certain duties from the trustees and must invest with the sole purpose of providing for the beneficiary (there was a crazy thread a while back where the mother and trustee wanted to pay off her mortgage with the trust she managed for her son!) Some people find this onerous.
Another option is to simply earmark a pot. And gift as and when you see fit on your terms. This way you retain full control and can keep the money if you change your mind. However, you miss out on tax allowances, growth/interest/income will be taxed against you, and lets be honest, in a lean year we may well decide to raid it to pay for the holiday or whatever with the intention to repay that never quite materialises. Plus, it will stay in your estate should IHT become an issue.
Lastly, pensions. Well its a nice idea. But the issue here is that young adults need help now. Cars, education and property are so expensive, and pensions cannot be accessed until age 57 from next year. By that point you'd hope that they'd have a career of their own and possibly inherited. Arguably the money is better used as a leg up early in their adult life.
In conclusion, there is no perfect answer, only best fit solutions for your objectives.
All options bar earmarking the child is likely to be able to access their money whether you want them to or not. The best option is educate them financially and give them the tools to make their own sensible decisions.