Hardwick will start at inside centre in a back-line including fellow Premiership player Marcus Smith (Harlequins) and Fraser Dingwall (Northampton Saints) for the tournament opener in Cork on Friday night.
Heyes, who has been a mainstay in Leicester's matchday squads this season as an understudy to experienced international Dan Cole, has been included among the replacements.
The pair will feature in their second U20s Six Nations tournament after featuring throughout last season's competition.
15. Josh Hodge (Newcastle Falcons), 14. Ollie Sleightholme (Northampton Saints), 13. Fraser Dingwall (Northampton Saints) - captain, 12. Tom Hardwick (Leicester Tigers), 11. Cadan Murley (Harlequins), 10. Marcus Smith (Harlequins), 9. Sam Maunder (Exeter Chiefs), 1. Olly Adkins (Gloucester), 2. Nic Dolly (Sale Sharks), 3. Marcus Street (Exeter Chiefs), 4. Joel Kpoku (Saracens), 5. Alex Coles (Northampton Saints), 6. Ted Hill (Worcester Warriors), 7. Aaron Hinkley (Gloucester), 8. Tom Willis (Wasps)
16. Ben Atkins (London Irish), 17. Bevan Rodd (Sale Sharks), 18. Joe Heyes (Leicester Tigers), 19. James Scott (Worcester Warriors), 20. Rusiate Tuima (Exeter Chiefs), 21. Ollie Fox (Yorkshire Carnegie), 22. Cameron Redpath (Sale Sharks), 23. Ollie Lawrence (Worcester Warriors).