There are many problems with your code as mentioned in the comments.
To make a better implementation of your parent/child form interaction with the comport component, do as follows:
Create a TDataModule
(ex: DataModule1), put the comport component there.
Now you can access the comport component from the main form and the child form.
Add a private method to your child form:
procedure TChildForm.ComPort1RxChar(Sender: TObject; Count: Integer);
var
data: string;
begin
DataModule1.ComPort1.ReadStr(data, 5);
Self.Memo1.Text := Self.Memo1.Text+''+data+'';
end;
When you open the comport in the child form, set the comport OnRxChar event to your TChildForm.ComPort1RxChar method.
In the TChildForm.OnClose event, set the comport OnRxChar event to nil and close the comport.