Please tell me there is a function for this.
I am stuck in a Garrison Mission to unlock the Menagerie.
I used the leveling stone so I have one level 25 pet.
I stuck two other pets in the newly unlocked Battle Pets Slots, they are level 1.
I evidently have to win these battles to continue to be able to level my pets.
I cannot win a 25/1/1 vs. 25/25/25 fight. It is evidently required to finish beating these 5 battle masters.
I cannot level my 1s (it’s all I have), no other battle masters will engage. They do not give a “Let’s go” option. Only the 5 battle masters do.
Abandoning the quests does nothing to change this.
I just need to remove these level 1 pets from the Battle Pet slots to make it 1 v 1 @lvl 25 again.
No, they do not have a cage option.
Tried addons to set empty teams, nada.
Any help would be greatly appreciated!
Welcome to Pet Battling! 
Please take a breath, take a step back, and relax. 
You are not stuck in anything at all. You just think you have to do things you don’t.
You are misunderstanding something, but I’m not exactly sure what.
No, no you don’t. There is nothing you have to do to continue levelling pets. There is never any forced gate to that. You can level them fine right now in any number of ways.
No. No, it’s not. I think you mean the quest Battle Pet Tamers: Warlords
That is just a weekly quest, with not a great reward, so not many people do it these days.
You mentioned addons. The two MUST-have addons are:
Rematch
Battle Pet Breed ID
and maybe PetTracker. You may want others soon, but these are great to have from the start.
You never can empty your pet slots - remove pets from them entirely - but you can drag and drop other pets from your collection into those slots. You’ll be doing that a lot. I see you have a handful of Level 1 Greens. https://worldofwarcraft.blizzard.com/en-us/character/us/azuremyst/deathentaxes/collections/pets Try practicing that now
You are a complete starter at pet battles, who has just got one Level 25 pet through the Garrison. Ignoring the Level 25 pet for a moment, this is the ideal video for starting with Pet Battling. I suggest you watch it. It’s friendly, but with a LOT of info, and will do a lot to get you started, and with your Level 25 you have a huge head start.
https://www.youtube.com/watch?v=e-PJIrVzvFg
Another good thing for you to would be the quest Oh Where Oh Where Can He Be in Valdrakken, Dragon isles
Which is a short series of quests that will net you another three Level 25 Blue pets.
I can give you lots more information and suggestions, but I don’t want to overload you TOO much, so I’ll stop there for now.
Wowhead and
Xu-Fu https://www.wow-petguide.com/ and
Warcraft Pets https://www.warcraftpets.com/
also have lots of information and guides if you want more reading,
Please have a look at this much, and come back with any questions! 
I’ve tried other Stormwind pet dudes and a combat option does not show in their text.
However, you have a point. I didn’t touch pet anything until I was well into the Garrison, and I have only been playing WoW for 3 weeks.
I’ll take a step back, abandon those quests for now, and start from “Day One, you and your pet.”
Let’s see if I can get things rolling, and thank you!