here are two cases mention below a) 1- First if the custom object have a master-details relationship then the child security will always follow the parent. b) 2 -And in second case when child is a standard lookup relationship, then the child can have it's own security model.

@Dileep Thank you, could you please provide me with some examples.

Sure . Some standard objects, like Contact and Activity, have the special option Controlled By Parent. This option grants the same access to the record as the user has to their parent record. If they own an account, they have read and write access to all of its contacts - If they cannot view an account, they cannot view its contacts. The checkbox on the rightmost column means that access rolls up through the user’s role. If you own an account, your manager has the same access to it as you, and so does their manager. It is important to note that standard objects will always have this option selected, meaning you cannot disable role hierarchy sharing for standard objects like Account and Case.

@Dileep Thank you for sharing in detail and I got it.