Not having any kids myself I haven't looked into this particularly hard, I just want to add that /r/ukpersonalfinance has a bunch of horror stories from people who at 18 got a letter telling them they now have access to £x and who then spent it all on rubbish they later regretted. Hopefully children growing up in an MMM household with lots of financial literacy and skills would know better, but if you have a specific plan in mind like a house deposit, I'd bear in mind that it may be safer in your hands than theirs.
Whether the tax advantages outweigh that risk I don't know, presumably it depends on what your alternatives are, whether you're using up your personal ISA allowance etc.
Sometimes I wonder if there should be such a thing as a junior LISA!
That is perhaps the baby bond stuff? Not sure.
We were doing similar things in Canada - RESP and in trust for accounts, but it's true the RESP remains in control of the 'subscriber'. The in trust for becomes the child's money when they come of age but I don't think anyone sends them anything to notify them!
The nice thing about the JSIPP is that you can't get it til (state pension - 10 years) age. If you haven't become sensible by then (and let's be fair, they would quite likely already have had an inheritance from me by that point), oh well.
The JISA. Yeah, well, I'll have to risk it. I'm going to be shovelling cash into my ISA and SIPP as fast as I can, but this extra I already consider 'their money'. I don't want it on 'my books' if that makes sense.
JISA is for university or house deposit or whatever (early retirement jumpstart!); when they are 18 it is up to them though yes of course I'll be talking them through it well in advance.