If your child is over 18 then, yes you can.
If your child is under the age of 18 when you make your Will but is over 18 when you die, that is okay too.
However, you can appoint your child as Executor, even if they are under 18 when you die, as long as they are appointed alongside an adult Executor.
Section 118 of the Senior Courts Act 1981 states that you cannot act in the administration of an estate until you are 18.
However, Rule 33(1) of the Non-Contentious Probate Rules 1987 states that if a minor Executor is appointed alongside an adult Executor then the adult can act in the administration with Power Reserved to the minor. Upon the minor reaching the age of 18 they can then apply to the Court for a Grant of Probate themselves.