He's debating whether to get either a trailer, or a tagalong / trail-a-bike so she could ride attached to this bike. The main question is, which is likely to lead to her being willing to take part in longer rides? Tagalongs are easier to get used and are less expensive, but that doesn't mean they're the better solution.
She is very confident on her balance bike (glides long distances), and my guess is that she will be ready to transition to a two-wheeler pedal bike in the next month or two. She's 40" tall.
The two of them borrowed a trailer today and took it for a test ride. She liked it ("especially that it has pockets!"), but of course, it was an exciting, brand-new experience, so we have no idea how long she'd last once the newness wore off. She no longer takes naps.