For a CP ressie in MK we were let in with a group around 7:45 am; at DHS H&V ressie, we and the crowd of prepark diners were let in at 7:55 am. Both parks were to open at 9 am. With the crowd to get in we were late for H&V ressie but just made our 8:05 am ressie at CP. Note that they gave out pagers at both places after you check in at the podium; at that point I suspect it is first come first serve, but I'm not an expert. My advice is to get early to try to be near the front of the line when diners are admitted and get through bag check.