Child abduction is when one parent moves their child from their usual home to another country without the agreement of the other parent or of anyone else who has parental responsibility (or without a court order allowing it). It can happen when a parent wrongfully removes a child to another country or wrongfully retains a child in another country.
Even if this has been done by accident, with the parent simply being unaware of the law, it is still considered child abduction.
If you and your ex both have parental responsibility for your children and there are no court orders in place, then neither of you can take the children out of the UK on holiday without the consent and written permission of the other (or anybody else who has parental responsibility for the children). Many parents do not know this.