If you kick out someone close to midnight, the probability is high, that some of their other Pokémon are already kicked out of different gyms and they already got their 50 coins for the day. So in this case it would be more "polite" if you wait till after midnight to kick them out---so they can get 50 coins again.

Though I personally wouldn't stress so much over it. It's all part of the game. If you're not actively griefing, kicking everyone instantly out and hogging all the gyms, it's okay. (I usually check how long the Pokémon is in the gym. If it is in there for a short time---less than one hour---I let it stay.)

One time I started taking a gym and after 3 minutes I saw a 20+ year old man sprinting down the street, yelling at me. I was on a bike so I just biked away because I did not feel the need to get stabbed over pokemon go.

So, don't do that and you will probably be fine. If it's a gym close to your place and you notice that it gets taken at the same time every day you could shift your habits around so that you both get a couple of hours. My dad is a pensioner and he grabs a gym each morning walk and some kids grab it back on their way home from school. Nobody get a perfect coin pay from it but they get fair amounts. The push and pull of gyms is part of the game

"Losing" is part of both the game and life, the kids will get over it. I mean, as kids we played games like TMNT and Battle Toads and we managed, not getting 50 coins because you got kicked out of the gym is nothing on comparison.

But yea, if you want to be extra nice, you can look out for these things and lose all your PvP battles.

Honestly, if you care about kids' experience that much, join them in raids, that does much more.

I haven't played in years but, back when I played, the etiquette was similar to what everyone else has said: if it's been in there less than an hour or so, let it stay. And if you aren't going to be around to defend your gym and don't especially need the coins, put a 10PP pokémon in there so it's easier for a local to kick you out later.