Prologue
John and Sherlock got married, it was a great wedding. Everyone was there. John's old army friends John's parents; hell even Harry went. Sherlock's family went, obviously and John got to meet the delightful pair for the first time… that wasn't sarcasm, they were delightful and were pretty normal. John's best man was Mike and Sherlock's best man was Lestrade.
Around three months after the wedding Mycroft went round with some information, at first Sherlock thought it was a case until Mycroft told him it wasn't but it required Sherlock and John. Once they were both there Mycroft began to explain; they'd heard of using one person's DNA in an eggcell to clone but they'd learnt how to half their DNA and put it with another person's half and then put it In the empty eggcell. Mycroft then explained that it was around 3 grand to get done. John asked why he was telling them then. His exact answer was "I'm not suggesting that you pay, I haven't gotten you a wedding gift yet, I know Sherlock wants a child but I don't know about you."
And that was how they got Hamish. He was technically a wedding present. This story is set around six years after Hamish was born.