He doesn't know how he got to this point and he doesn't particularly care because at the moment Bilbo is underneath him, clutching at his shoulders with tiny hands and squirming with intent. Bilbo isn't squirming away, so Kíli takes this as a good sign and continues to unbutton the little hobbit's shirt but even as he does this he can't help but smirk to himself. Little. Kíli's hardly any taller than Bilbo, but still...
Kíli knows this is not the time nor the place to do such things, with a hobbit nonetheless, but seeing the naive hobbit attempt to shoot an arrow and getting knocked back on his butt, well... Bilbo looked awfully adorable on the ground getting his little trousers well dirty and... How could Kíli resist the urge to join him down there? He was used to getting dirty, after all.
But Bilbo is fidgeting and wiggling out of his clothes and he is just too prim and proper, being willfully stubborn about taking his time. Kíli has to suppress a laugh when Bilbo tries to fold his clothes properly before getting down to business, but Kíli grabs his tiny wrists and pins Bilbo's hands above his head to incapacitate him. While the hobbit was busy taking his time Kíli had already gotten out of his trousers, shucking them away into the dirt.
After that it's all downhill and Kíli certainly wastes no time in making Bilbo his. Kíli knows he should've done something to ease his way into Bilbo but he simply couldn't wait any longer or else he would've gone mad. So when he initially enters Bilbo's body there's a harsh intake of breath on Bilbo's part and there are small hands grappling to Kíli's chest, struggling to stay grounded as he's breached roughly.
Bilbo can't breath for the longest time and for a moment he thinks he might pass out from the lack of oxygen but Kíli leans down to brush his lips across his nose, making him let go of a shaky breath. The motion is far too intimate for the pain he's feeling right now but seems oddly appropriate when he looks up into Kíli's dark warm eyes. His face is obscured by the afternoon's shadows and hair that has fallen over his shoulders, tickling the sides of Bilbo's face as he goes in for a kiss.
Kíli has stayed perfectly still this whole time and when he finally pulls back Bilbo hisses under his breath as he re-breached by Kíli's cock. Bilbo's chest aches as it clenches, forgetting how to breath, and Kíli stops as worry creases his face. He really should've thought this through better. Bilbo squirms slightly underneath Kíli and Kíli thinks that maybe Bilbo is having second thoughts, so when Bilbo readjusts himself and pulls the bow that he's been laying on away, Kíli sighs with relief.
Bilbo lets Kíli pin his hands down above his head once again and manages to keep his breathing steady as Kíli moves within him and Bilbo is just too small, even for Kíli. Bilbo is too tiny and impossibly tight and Kíli has to push harder than he wants to, but Bilbo seems intent on letting Kíli have him on the dirty ground. Kíli's dark eyes flicker over Bilbo's face and when Bilbo has seemed to relax a little he tries moving again, this time with success.
Bilbo's breath hitches and his body arches up into Kíli's, shaking all over. It still hurts, and it should, but Bilbo lets a small moan escape his curving lips as Kíli thrusts as gently as he can into Bilbo's body. Kíli releases Bilbo's wrists to cradle the hobbit's delicate face in his hands, sealing his lips to Bilbo's in a wet kiss. Kíli's facial hair scrapes against Bilbo's smooth and deceivingly young face, eliciting more tiny sounds out of the hobbit.
It's surprisingly tender and almost sweet that Kíli is being so gentle with him despite their rough coupling. Bilbo wraps his arms around Kíli's neck so they're pressed closely together as Kíli begins to thrust in and out of him, hanging onto the dwarf with everything he had. He's panting softly into Kíli's long hair and he's well aware when one of Kíli's hands leaves his face to touch him. Kíli's hand is so rough but so gentle as it slides along his length, drawing inhuman sounds out of Bilbo that excite the hairs prickling along his neck.
It's getting dark and soon someone will come looking for either Bilbo or Kíli, but even as both are aware of this neither of them care. They only care about reaching their climax in eachother's bodies and when Kíli is so gentle and agile, Bilbo can only hold back for so long. Bilbo struggles to keep quiet as he comes undone by Kíli's hand moving around his cock and he almost misses it when Kíli reaches climax within his small channel.
They're both gripping at eachother for the longest time before they finally still their labored breaths, panting into the clustered space that remained stuffy between them. They realize that they haven't said a word since this started and now that it's over they still don't feel the need to speak or justify what's happened just now. So instead of words that could warp this otherwise pleasant moment, they decide to kiss eachother instead.
Neither dwarf nor hobbit complained.