"Up you go!" is not imperative. You cannot use it as a command.

This is better said in context. Saying *Go on up* is like inviting someone in a similar phrase is *Go on In* and one more *Come on up* or *Come on in* You’ll hear this in an inviting way. Like if someone has a upstairs and you're waiting for them they might be doing something. They will stop and say.. *Go on up!* or *You can go on up!* Meaning "Go up staris no need to wait for me" something of that nature. For *Go on down* Another example: Say you have an electrician, the lights aren't working down in the basement. You let the electrician in. You both walk to the basement door while speaking Your kid comes up to distracting you And the guy just stands there. You look at them saying, "Go on down," or "You cam go on down" giving them permission while you tend to your child and follow him right after. As for *Up you go* I love this one. You'll most likely hear this around a child. Say a mom is putting her baby in her highchair She will pick up the baby and add a cute little *Up you go* It's kinda like baby talk. 👶 And when she takes the child out of the highchair she will say *Down you go* again baby talk. But... you can also hear it with adults liftting other stuff than babys. IM SO SORRY this was so long but I hope it helps😭😭