My understanding is I need to open my own TD account before I can open a JISA account for my child.
OIC's state below that any account that has a balance less than £5100 will be subject to £30+VAT yearly charges. I don't plan to invest in anything, I purely want to open a JISA for my child. Am I right in thinking the below applies to me? Or does it only apply to someone who has an account and is actively investing?

Online Investment costs
£0 account fee when you either have an ISA balance of £5,100 or more, or have a Regular Investing (monthly investment) facility set up. There’s a £30.00+ VAT per year or part year if not. Share dealing and other rates and charges.

I hadn't realised you needed an account yourself to get a JISA. JISA products with other providers don't all have that requirement. But makes sense for TD's business model given a JISA has no fixed annual admin fee (other than the platform fee for holding funds) and they don't want a bunch of dormant low value accounts. HL have higher fees generally, so are keen to hook people in with accounts for kids without too many restrictions.

If you wanted your own TD ISA account, then as you noticed you can avoid the £30+vat annual charge even if you don't have £5k in it, simply by having the account set up for regular investing which is minimum £25pm.

If you don't want your own s&s account, why not? Putting away at least a couple of hundred quid a year for your (or your family's) medium to long term future sounds at least as useful (probably more useful) than putting a similar amount into a JISA which can't be accessed until 18.

As a footnote however, TD Direct recently announced they are being sold to Interactive Investor, subject to regulatory approval. This means the quality of service and fees may ultimately change to something else, over some indeterminate timescale. I have been an advocate of TDD for years as their broking side is good and full-featured, but i can't say I'll be with them in five years as they may well change the frontend, middle or back end at some point after sale.

Other groups like HL and CSD are not currently known to be up for sale, but in reality there is consolidation in the industry all the time and every business is for sale at the right price, whether they are stock market listed or privately owned.