It's possible to allow your managers to validate the real hours of their teams without giving them the possibility to validate their own hours !
As an owner or director, go to your manager's profile: at the bottom of their profile, check or not the option "Can validate his/her own actual hours".
What is the impact for your managers?
When the option is on, your manager can validate his real hours as he does with the employees.
When the option is off, your manager can declare his worked hours, i.e. give the indication of the worked hours, but you will keep the hand to validate his real hours.