Change of name deed for a minor
You can use this to change the name of your child (i.e. someone under the age of 18) by deed poll. The child must be a British or Commonwealth citizen who lives permanently in the UK.
To use this, you need to have parental responsibility for the child. If there is another person who also has parental responsibility, they must agree to the name change. You can't use it if there are 2 or more other people with parental responsibility.
Note that although you can use this to create a valid deed poll, it's not suitable if you plan to enrol (i.e. register) it with the Royal Courts of Justice. Enrolment often isn't necessary, but may be in some situations.
If your child is aged 16 or over, they must agree to the change of name by signing in both their new and old names. Note that if they're 16 or 17 and are (or have been) married or in a civil partnership, you'll no longer have parental responsibility for them.