We’re the Department of Official Agency. So there, neener! 
IOW: DOA.
Forget about the boss supervising themselves. I’d bet this is part of some sort of short fuse crisis, and the boss is probably busy doing last minute reports and stuff.
It could be that the OP has a history of slacking, but if that was the case the OP would probably not post this, so we can exclude this case.
My guess is that the boss has screwed up something in the past, and so wants to make doubly sure that they are not going to screw up this one, and thus pushes the pressure downwards.
Oh, and it would offend me for sure. Especially because I’m guessing that the assistant would have no idea about whether the OP was doing the job correctly.
It would certainly be good to tell the boss that this supervision wasn’t appreciated when things calm down. You might get an apology. I hope.
/snark
To babysitter: Use this chair. If it isn’t comfortable enough, I can find you another. Would you like to put your feet up? I can get you a cold drink. Is the can okay, or would you like it in a glass with ice? Anything else you need?
When assignment is completed, hand it off to the babysitter, then follow the babysitter to the boss’s office. Watch the babysitter hand it to the boss, then wiggle your fingers: Hi! I just had to make sure the babysitter brought this directly to you.
/endsnark
Would I do this? Nah. But while I’m completing the task in my typically excellent way, I’d have a helluva smirk on my face.
I once had a boss who was capable of horseshit like this. When I had enough of it, I looked for a new job where my talents would be appreciated, not questioned.
~VOW
Not even really snark.
No, I wouldn’t perform in this exact way. Having some medium faith that normal humans can detect sarcasm and react in wild, unpredictable, erratic ways.
But, yes, in broad strokes, yes.
I’ve not been in an analogous situation that I can recall — similar, but not identical to the OP — but changing the dialogue a smidge, I think that is the correct response.
Provided the task(s) have been performed correctly.
This is a correct manœuvre, IMHO, and one that I can see being performed not only by me but by any other conscientious worker who takes responsibility for his or her work.
After all, child-boss was merely observing, perhaps as part of a training exercise…actual worker performed the work and is solely responsible for its contents and quality. Probably a reasonable assumption.
And if the “boss” is at all sentient, he or she will have recognized what a charade he or or she wasted time upon. Bonus.
I do wonder whether the OP’s work situation is such that someone else might possibly interrupt him for some reason while he was performing this crucial task, and the lackey was actually there to fend off such interlopers.
So, @Grrr, did you talk to your supervisor about this?
And if not, why not?
We’ve expended a lot of effort guessing at stuff you could resolve with 3 minutes of frank conversation.
If your boss is a frank converser
And if not, they’re a useless lying shit who it’s pointless to speculate about since they could do anything or nothing next time for any reason or no reason.
No, it was a one off incident. Other than this we have gotten along pretty well.
Review time is near and I KNOW he is going to advocate for me to get a nice raise.
If it continues to happen I’ll say something but right now I’m just going to let things be.
It could be that there was a previous instance where someone, not the OP, didn’t get the thing done, and so the boss vowed never to have that happen again and assign 2 people to make sure it got done. In other words the OP is paying for someone else’s mistake.
Given that it sounds like the OP is on good terms with his boss, I think that they should bring up with the boss that it rubbed him the wrong way. Not in an aggressive FU way but in an FYI way, so that he knows how it comes across for future reference. Otherwise he might continue the practice with other employees.
But still, there is the disrespect that the boss didn’t tell Op what was going on with the babysitter.
“Hey Grrr, I know it sucks but Big Bossman told me I needed to make sure you finish on time. Not my choice and I know you’ll do it, but I gotta cover my own ass with upper management. Hope you understand.”