Only Madeleine's parents can apply to have her declared deceased. And they have no need to, nor are they going to.
Also any request could very well be denied, which it almost certainly would be. Especially since there would be no purpose to such an application.
You're an early bird, Eleanor!
It was an interesting point of Snartibartfast, but I don't see the material relevance either. Unlikely the McCanns would do such a thing.
Isn't it normally something that is done when adults go missing?
I have a good friend whose brother went missing (still unfortunately not found, after 12 years), and the family went through a long fight with banks, a mortgage lender, insurance companies etc., who withdrew monies from his bank account for a long time after he went missing on the grounds that legally he was still alive. Terrible.
The family couldn't sell his house or arrange his financial affairs because his effects weren't theirs to sign over. It took years of legal help before they got some financial relief, and eventually at seven years finally everything was concluded.
The seven years dead law also comes into the case of the desire to remarry in event of a spouse's disappearance. A common problem in wartime when men were lost without trace.
Don't see any relevance of this law for a child, however.