So, how deep of pockets does Gramps have? Wouldn't want the grand daughters to be scarred for life by forcing them to ride a high-ten bicycle.

Keep in mind, the step-through bikes are a nice "vintage" style, but if they are wearing pants or shorts, no reason they can't have horizontal top tubes.

Here is a cute Terry-style Bianchi.

Here is a sweet Merckx frame. Cantis. Doesn't specify the wheel size. How much are your granddaughters worth?

Schwinn Traveler, Lugged 4130 Mixte Frame

Schwinn World Sport, Lugged True Temper

She Devil... new chromoly step-through. At least one granddaughter needs a "She Devil"... right?

Dancelli Step-through with cantis

Telaio corsa donna acciaio Villata vintage steel road women frame Columbus SL Ohhh... SWEET!!!!

Hugo Rickert Damen Rennrad / 51cm / light blue / Reynolds 531 Frame Fork Vintage

I think there may be some Step-Through Colnago Super and related bikes out there... somewhere. Not a lot, but a few.