Paying tax on interest over £100 on children's savings account

Jazzking

For the tax year 2024/25 my son (12) will have earned over £100 of interest from a HSBC savings account in his name.
The capital in the account came from a Halifax Kids Saver account opened and operated by me in trust for my son since he was born so am I right in thinking that the source of funds in the HSBC account is from me and therefore the interest generated is liable to be taxed at my marginal tax rate?

For the same year, for the first time, I will have breached the £1000 PSA limit for interest earned on my own savings.  My HMRC tax account isn't showing my breakdown yet for 24/25 but I'm fairly sure my own records I will be at around £2.3k of interest.  I've not had a tax code adjustment from HMRC yet and I don't do a self-assessment return.  

Ideally I would wait for HMRC to get the interest info from my various banks and work out the necessary adjustment.  However, I'm thinking they won't factor in the interest earned on my son's account so I'm going to have to contact them to declare that anyway - is that right, in which case they will need to process details of my own interest (do I just tell them my figures?)

I didn't think I'd have to do this, but with rates the way they are for kids accounts (5%) able to hold up to £5k it will end up being quite common that this interest will be taxable - assuming the capital came from a parent.

I'm aware it's not an issue for JISAs and I have a S&S JISA on the go as well for him, but may well start to explore a cash JISA too although the rates aren't quite as good, even after tax deducted,

xylophone

https://www.gov.uk/savings-for-children