I do believe they were seriously wrong to neglect madeleine in this way prior to her "disappearance"
I use this term to negate any complicated expressions of subjective views, as we dont know for sure, whether she is dead, or isnt... etc
The police etc did take a bit too long to do certain aspects. While fault can be predominantly lain at the feet of the parents, some needs to be spread to failures of the police (ill highlight certain aspects later on)
Under the Rule of Law, they were under the jurisdiction of spanish and eu law, not english law. any offences relating to the individual madeleine incident would come under the former not the latter, and i am not 100% sure of the spanish law personally to make comment on what charges when all resolved they would be charged with.
However, in light of their actions, it can be brought up indirectly in the uk courts on english law, for their possible dangerousness in child neglect etc thereby relating to the twins and possible removal from care (from their example of maddie)
Whether this would succeed or not, is not something i cannot fully speculate, however, what i do know is if it is brought up in the english courts for a case of child removal, the courts would need conclusive evidential proof, that ever since that time (in any form) the parents have ALSO neglected etc the twins. The prior basis of madeleine would then be weighed against them.
Depending on whatever result, the EU courts would also end up being involved, due to the evidence basis on madeleine that they "can" do neglect and have done so in the past - whilst in the spanish jurisdiction specifically. This is also due to EU law being supreme to all members of the EU, and thus we would have to comply.
If they have not neglected the twins (under any specified requirements of actus reus) in english law, then I am not sure they need to be removed right now, but i do feel, that they should be more observed, that mistakes of the past are not repeated