Savings advice for child

[Takemeaway]

I’ve a 15 year old and 13 year old who have borne received about £6k

Where’s the best place to lock it in till they’re 18?
Not sure the JISA is the best option

[DaveH23]

Why not a JISA.

Make sure it's S&S not cash ISA. Pick an S&P 500 index and forget about it.

[Mr Squarekins]

Takemeaway said:

I’ve a 15 year old and 13 year old who have borne received about £6k

Where’s the best place to lock it in till they’re 18?
Not sure the JISA is the best option

I'd genuinely show them at this young age, the magic of compound interest. If they don't need the cash, since it appears to have been a windfall, show them what it'll be worth in, say 10 years and 20 years.