Assalaamu alaykum dear brother,
Like any other investment, a CTF or Junior ISA needs to be Shariah compliant. There are Shariah compliant and non Shariah compliant products out there.
Those ISAs and CTFs which are cash based in the conventional market are 99% accruing interest.
The shares junior ISA and CTFs need to be screening for Shariah compliance.
Therefore, I would recommend you to look for a Shariah compliant Junior ISA.
Allah knows best