No, you cannot carry over or use an unused JISA allowance from a previous tax year. JISA allowances operate on a "use it or lose it" basis. If you do not top up the full amount permitted under the allowance by 5 April, the remaining allowance is lost forever and does not add to the next year's limit.
The Freetrade stocks and shares Junior ISA (JISA) is coming soon! Keep an eye on your emails or theFreetrade website for updates.