Disclaimer: I do not own either of these animes and I never will, so don't sure!
Setting: After Kenichi starts training as a disciple of Ryozanpaku in the anime/manga series Kenichi: The Mightiest Disciple and after Tsuna and them have returned to their time after Tsuna has defeated Byakuran in the TYL future, but before the Inheritance Ceremony arc in the Katekyo Hitman Reborn anime/manga series.
Tsunayoshi Sawada, or Tsuna for short, has gone to visit his cousins Kenichi Shirahama and Honoka Shirahama and their parents for Namimori High School's spring break vacation. Reborn, his home tutor, of course, has gone with him. The rest of Tsuna's Vongola mafia family are scattered all over for their vacation plans.
After staying a while at his aunt and uncle's house, Tsuna, along with Reborn, asks Tsuna's aunt, who is the younger sister of Tsuna's mother Nana, where his cousin Kenichi is and Tsuna's aunt tells him the truth that Kenichi is now living at the dojo he is studying martial arts at, called Ryozanpaku. Tsuna nods and decides that he would like to go there and check the place out, the next morning.
Author's Note: This was only the prologue, so I know its kinda short. Btw, the reason why Tsuna is the older cousin is because I put him that way