That's really interesting, Jess. I did think there was some sort of hard and fast rule about this think but the idea of following my instincts does actually make sense, on this matter.
Feeling a bit less niggled now. Mind you, i think i've got some major tweaking to do now, on the second edit.
Returning to the original question, I would go where you want the emphasis to be - if you want the reader to hesitate slightly, thereby emphasising the middle line, then have the final word of dialogue on the next line.
With children's writing, I've found, we really have to work at these breaks as we have normally limited words and anything that can be made to stand out without words is good. Also going for an uncluttered page presentation.
Would agree it is a subtle but important distinction.
Personally I favour the second option. Not sure why. Makes the dialogue more immediate I guess, so there's no big pause between the looking and the speaking. Also looks neater. But that may be a weird form of writerly OCD I have!