Let's say hypothetically that Kinemon is not Momo's dad but bodyguard.
Why would Kinemon abandon his duties to protecting Momo who is an important figure of Wano(probably the Shogun's son) to voyage with the Strawhats? Kinemon abdicating his post would weaken Wano. I would imagine that you don't just put anyone on protecting the Shogun's son. The 3 samurai assigned to the task(Kinemon, Kanjuro, and unknown one) are probably some of the strongest/proficient if not the strongest samurai in Wano. We also haven't seen Kanjuro or the other samurai's personalities yet. They could stand out and be just as worthy as Kinemon…which in a sense would eliminate all 3.
Also if you are a bodyguard, are you going to let the person you are supposed to protect out of your sight for an extended amount of time(months to years)?
This question is directed to Momo joining the crew. The Shogun is just going to let his son travel with pirates without having some of his most trusted men who his known X amount of years with strangers(Strawhats) that the Shogun may befriend in probably a couple of days One Piece time? I think not!
We don't know anything about Kinemon, so Oda can create some type of dream that he couldn't fulfill because he has been protecting Momonoske for so long and as you have seen those guys are all about duty. Momonosuke is growing as we speak and becoming brave by the time the Wano arc is over Momo won't have those mental problems he seems to be having right now and who is suggesting Momonosuke?
Do Kinemon really look like one of the Strongest Samurais in Wano to you?