Which children's bank account do I need?

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I've been saving for my child for a while now, in a separate pot with Monzo. I'm wanting to open a separate account for her but ideally one that doesn't automatically transfer to her at 18. Is this possible?

In short, No.
Any account in her name, is hers.
If you want to retain control once she is an adult, then you need to keep the money in your name. You could use a fund within your Stocks & Shares ISA for an investment which you are making as being for her, as an example. It is still in your name though, so you decide when you Gift the money to her.