We got a Specialized Hot Walk for DS. He didn't have long enough legs to use it until he was after 2 years old and he STILL can't handle a pedal bike (At 5.5) though he LOVES his balance bike.
Married 3/04
DS 8/07
DD born 8/11
Thanks for all the helpful info! Sounds like a balance bike might not be ideal given the slope of their driveway (gentle slope, but downhill towards a rather busy street). As I remember trikes, those aren't as easy to pick up speed unintentionally. Since DGD2 is probably too small now, anyway, I'll just keep an eye on the situation and see what DIL wants to do as she gets older.