Why is Son Gohan always so strong?
I mean Son Goku trains all day long, but Gohan studies all day long. So how did he manage to be so strong when Cell or Buu arrived? I mean, both started training when they were kids?
Is it same with Son Goten? I mean Fusion SS3 at this young age, this is quite strong.
Not sure which episodes, but it is explained a few times that each generation of saiyans grows stronger than the previous generation. This is the reason why Frieza feared them as they will eventually get stronger than him. This means if both Goku and Gohan didn't train and were the same age, Gohan would be naturally stronger. It's the reason Gohan tends to catch up quicker in power levels since he is more naturally gifted.
It is also interesting to note Vegeta is from a more genetically gifted line (royalty was the strongest saiyans) so he would be naturally more powerful than Goku if all conditions were same. Vegeta also points out to Gohan in buu saga that he has been neglecting his training so he was able to catch up to Gohan's power level when he defeated cell. Goku like you pointed out just trains like a beast even when he is dead so he tends to eventually surpass everyone.
If you had seen the seasons completely, there is an episode (I think it was in the Namek saga) where Vegeta looks at Goku and thinks to himself that the reason to Goku's true inner strength is due to an Earthling family. I think it is implied that such children are also far stronger than pure Saiyans.
I am relating these bits of information and would say that children born with Earthling side are able to access their powers without much effort.
According to Vegita, it probably is because mixing saiyan and human blood makes stronger hybrids
Vegeta: “At any rate, the battle power of Kakarot’s son is unusually high,
even by the standards of Saiyan children.”
Nappa: “Maybe his reading was wrong.”
Vegeta: “No, it wasn’t wrong. Raditz really took a large amount of damage from that brat’s
attack. It seems that mixing Saiyan and Earthling blood begets a powerful hybrid.”
Gohan's hidden power was talked about from almost the very first episode of Dragon Ball Z until nearly the very end. As far as he goes, his strength comes from the fact that he was thrown into fighting life and death battles when he was just 4 with the arrival of Raditz and his training under Piccolo. Just look at his resume: Raditz, the Saiyans, Frieza and all his minions, and then Cell. I stop at Cell because that was the last time he seriously trained for any sustained amount of time. But even after that, he stopped training for 7 years and his power decreased to the point where he was "only" a little stronger than Perfect Cell. During the Buu saga, the Old Kaioshin just unlocked the rest of his dormant power. So with all he experienced when he was a kid, it's only natural that he'd be a formidable fighter regardless of how much he's let himself go.
As far as Goten goes, he was being trained by Chichi. She might not be very powerful herself compared to the Z fighters, but she's certainly the strongest female in the world aside from 18. Her training methods were never really discussed but from the one flashback of her sparring with Goten, we can see that she was going pretty hard on him so it wouldn't be too far fetched to think that she'd be able to make him powerful. Like Gohan, he has the hidden power, so it's just a matter of training. The main difference is that Chichi allows Goten to train whereas Gohan had to either sneak off for that or there had to be something threatening Earth.
Sorry for the long explanation but I hope that helps.
Goku is the main character in the anime, he dies many times be he is brought back because where would the franchise be without him? Take this as an example, the episode of dragon ball when he leaned kamehamheha his first try. He always was the strongest and his children are because they train with their already stronger parents who face stronger opponents later on. This makes it all relevant.
