Children’s bank account

[AllSheNeedsIs]

Hi,
I’m wondering if anyone has any advice of which type of account to open for my 3 SDC, (6yo and twin 4yo).
They have bank account (isa) with their mum, but we think this money has been spent. We would therefore like to open something only my DP can access on their behalf. They have some Christmas money (cash / cheques) which we need to bank, but don’t want this to go into their other accounts.
Anyone any suggests on what type of account to open?

This probably isn't helpful but last year my dad went round all the banks and made notes and compared them as to which one has the best savings account for kids. (He wanted to put away a bit for my two) out of all of them he chose NatWest.. can't remember why though annoyingly, there was something about their account which was superior to the others. Maybe go in and ask.
Ahhh and it wasn't a bank card it's like a book that you would have to take to the bank with ID if you wanted to take money out so it can't be spent willy.
Also it's in my and DH name instead of kids but doesn't affect our end of year earnings assessment..: