Making them stronger

They never abused them. It was sparring. The only time any one of them came close to actually hitting a child was when Vegeta and Trunks had a bet about if Trunks landed even one punch on Vegeta then he would take him to the amusement park. Trunks actually lands a punch and Vegeta gets so pissed off that he punches him in the face and then asks him not to cry as he was taking him to the amusement park.

I didn't call it training.