I would not put the Runners in with the Pekin drake until they are fully grown, especially since it is still breeding season in some areas. Your Pekin male is huge and could crush the Runner girls if he tries to breed them before they are fully grown. Growing ducks have very delicate legs and can be easily injured or trampled by a larger bird, especially an amorous or aggressive drake. Wait until they are fully grown and then keep an eye on the drake's behavior. Sometimes a big drake, or multiple drakes, will drown a young or small duck while trying to mate with her in the pool.
Instead, you could partition off an area for your young ducks next to your drake so that they could keep each other company without any risk to your youngsters. Then you could let them out for supervised visits.
Chel