I too agree with Patti, but most accounts, including ISAs, become the child’s on their 18th birthday, regardless of whether or not you tell them about it! The institution just writes a letter direct to them with the account details in, and if online, cuts off the parental login!
Maybe the Nationwide future saver is different, but having got 2 young adults, no other accounts that they have had (including stocks and shares ISAs) have allowed a parent to manage the account past the 18th birthday.
My view is to save in their name and gradually allow them to know about/manage the money in their teens. If saving into a S&S ISA, get them involved with investment choices as they approach their 18th.
My two already had a pot of money saved by us, when they unexpectedly got a large (almost 6 figure) inheritance at 15 and 18. It could have been a recipe for disaster, DS receiving that much in his name only weeks after turning 18, but sensible discussion about house deposits, not wasting it, drip feeding it into ISA (and letting him and subsequently his sister have involvement in fund selection) has paid off. Neither are remotely interested in wasting the money and indeed are keen to add to it. If anything, it has helped with their interest in savings, budgeting etc, rather then making them want to blow it all!
They have both had cars, but re paid the money into their inheritance pots from their part time jobs.
For those saving in your own name, think about what happens if you die. It will become part of your estate, plus may end up with your other half and the DC may never see it. There are so many stories around about second marriages and money going to subsequent spouses, that it is worth considering what is written in your will in respect to the ‘child’ accounts.