Last year, we bought a child-sized bike and removed the pedals. My three-year-old has been using it as a push bike (a.k.a. balance bike, run bike, or scooter) and learning how to balance. The plan is that once she's ready, I'll put the pedals back on, but I won't be adding training wheels. However, I don't want to put the pedals back on before she's really ready because it would be a pain to take them back off again.
She's now at the point where she can coast for about five seconds at a time, but she doesn't ride in a particularly straight line. She often asks for pedals, though I'm sure she doesn't appreciate the effort that will be required.
How do I know when she's ready and it's time to put the pedals back on the bike? What specific goal can I give her to work towards (i.e., "When you can do XYZ, then you can have pedals")?
Related question: What is the best method to teach a young child to ride?