The JISA allowance is set each tax year, letting you know how much money you can add to your child's JISA. The tax year runs from 6th April in any given year to 5th April the following year.
The JISA allowance for the 2021-22 tax year is £9,000.
Please note: if your child also has a Cash JISA elsewhere you can only contribute up to the JISA allowance per tax year across both of the JISAs. It's your responsibility to make sure you don't exceed this JISA allowance.