They should. However, we should understand that for all the failings of parents as managers of a child's money, a five year old child is also not going to make good financial decisions, and there does need to be someone who can make prudent, and indeed numerate, decisions on the child's behalf.

Children may not be able to make good decisions with their money, but many adults aren't either, especially when it is someone else's money (ie, their kids).

A child should retain full control of their own money up to a certain value. Above that, they should be assigned a fiduciary to take care of the rest.

"Assigned a fiduciary" does a lot of work there. Who assigns the fiduciary? How do we find a good one? Why does this fiduciary do this job? God knows that financial professionals are not exactly immune to misconduct.

I think we should maybe admit that this is just kind of a difficult space, where there's in some rare cases a lot of money on the line and there's not necessarily anyone who's perfectly suited to handle it.

How does this prevent these issues? Do you think a 5 year old child would not sign over all their property to their dear parents and/or manager if asked? It just increases the number of people who would try to swindle the kid since literally anyone interacting with the kid could do it.

No. Children are property.

To quote Lady Hale, that is a startling proposition.

Both can be true!