Why should they be Sam Fed's Misguided Children, and not the UPF MC? Personally, I think they should exist, as well as UPF Army, but am less certain about equivalents to the AF and CG existing as stand-alone services. More likely, they'd be part of the naval Starfleet structure.
Well, we know from VOY's "Thirty Days" that there is a such thing as the Federation Naval Patrol.
I myself like the idea that the Federation has its own Federation Army and other branches alongside the Federation Starfleet, but the canonical evidence seems to imply that Starfleet's unique force-projection abilities have mostly rendered conventional armies and other forces obsolete.
And it's not implausible that Starfleet might encompass multiple service branches. The Canadian Forces encompasses the Canadian Army, the Royal Canadian Navy, and the Royal Canadian Air Force, all under a single service force.