In order to be considered as fully resident in Spain, you must have registered on the Padron i.e be on the electoral register. Being on the Padron means a lot more than being able to vote in local elections.
Many benefits in Spain are based on years on the Padron. Usually a minimum of 5 years is required in order to claim disability benefit and other help as a carer etc. Remember to register your children too. Even though they cannot vote until age 18, they might in the future need to claim a benefit and if not registered as soon as they arrive in Spain they might have to wait to get something they are entitled to.
Citizen’s Advice Spain is very helpful when dealing with specific cases and could help you apply for a historical Padron ( getting your Padron back dated, though this is not easy to do).
With Brexit on the horizon it is even more important to be fully registered as living in Spain. Residencia, annual tax return, 720 tax return, which is due before the end of March. All of these things matter if you want to be “legally” here in Spain.