Not gonna lie. I'm not sure about your questions about the daycare center. However, I've used the Daycare center to level up Magikarps until I can rare candy them into a Gyrados and I haven't seen any adverse effects in that respect. But if you use the daycare center to raise them like 40 levels, I'm not sure how that would affect the outcome.
So, I have all the badges and are going to fight the four.
Now, I need to level some of my dear pokemons and I wonder if there is a good place for this.
Here's how I trained for the Elite Four. It works perfectly if you have the Amulet Coin.
Save RIGHT BEFORE you enter the first battle arena.
Fight the Elite 4, and make sure a pokemon with the Amulet Coin is present in the battle.
Fight Lorelei until you've figured out her strategy (because she can get kind of predictable). And as long as you can at least beat Lorelei without using any revives or potions, you will win 13,200 as your prize money.
Then, even if you lose against Bruno, you'll only pay about 7,000 as compensation. So you end up (a) winning a ton of money and (b) you get insane amounts of Exp Points. Which works great if you focus the Exp Share on one specific pokemon.
HOWEVER
if you don't feel like going through that trouble. Go to Celadon City. Go west until you get to the hill right before that huge incline with the bikers. And then use the VS Seeker on the bikers there endlessly. With the Amulet Coin, you win a little over 1,000 for each battle, and cumulatively win about 1,500 Exp Points as well. (This obviously depends on which version you're playing. I can only advocate for FireRed and LeafGreen).
I don't really like training on Victory Road. It's a bit too tedious and you don't win any money.
Also, what is EV and why is it so important to "train" it?
This is easily the best most concise place to quickly read up on EV training.
https://www.gamefaqs.com/portable/ds/file/925601/48969