Pick a weekday if possible. Last week of April / first week of May is a slower time.
Start in whichever park does not have early entry, which probably means staring in Studios. Diagon Alley will be low crowds for a little while after opening, or they can choose to head for rides that don't offer single rider or are slow single rider lines (Simpsons or Rip Ride Rocket come to mind). I would not do Minions first thing, unless they really want to ride it and they're right at the front of the pack. It seems Minions is what most people do first.
The single rider lines are great on most rides. However, I would suggest doing standby if waits are reasonable for Gringotts & Forbidden Journey. For any other single rider line, you don't miss much in the queue. Well, maybe Hagrids, but that single rider line isn't open all the time anyways. Rip Ride Rocket can have a very slow single rider line, sometimes Velocicoaster too.
They probably can't hit every single ride, but they can do the majority if they plan to open & close the parks. Should leave them wanting to come back!