It really is pretty much the same except for the terminology and some of the paperwork. We have a guardian instead of a trustee, who is holding the assets and managing them for the child, without the legal apparatus of officially setting up a trust. Taxation was very different in those days so there isn't an annual tax filing, but there's an annual court filing instead. Some states might still have an annual court filing today - federal tax law is the same everywhere but other aspects of estate and trust management are controlled by state law.
In the modern world, I simply don't see trusts for minor children in action because their parents are living longer. It's mostly surviving spouse trusts, which follow a similar concept to the provisions for widows that you posted. There's less concern about conflicts of interest nowadays, and most people appoint one of their adult children to be the trustee on behalf of their surviving parent. There's an obvious potential for abuse there, but I haven't seen it happen.
There were two opportunities to appoint a guardian for John Bishop Jr, once when his mother married William Marriott and again when she married George Proctor. We might be missing the records for the first time a guardian was appointed. If William Marriott was appointed as the original guardian before he married the widow, no one might have seen any need to change things when he did marry her. He would already have put up a bond, and if he was trustworthy before the marriage he wouldn't become less trustworthy afterwards.
If William Marriott wasn't the original guardian, there wouldn't have been a need to appoint a new one when he died. But the original guardian might have died too and needed replacement. Or maybe John Junior wanted to choose his own guardian when he was old enough to have a say in the matter.