Jesus Christ, it's ridiculous that a change of name can effectively wipe the slate clean like that.
It wouldn't be instant, as DBS forms require you to disclose any names you were known as before - I always skim over that bit because my birth name is my name lol, but I'd assume it's in the past five years, like with addresses?
Either way, child abuse shouldn't be so easy to hide, crimes should be attached to NI numbers or something else immutable. Branded on their foreheads, maybe.