Technically the height restriction does apply to the queue lines as well. This will be enforced by hosts at the ride entrance. However I'm sure that's only Wicker Man and The Smiler so you could get away with it on other rides.
You won't be allowed to wait on the ride platform, and I'm unsure if they'd let one of you wait with your child at the exit to do the swap, even if you've all queued. I'm thinking not.
The way I would do it is split up when you join the queue. One adult go ahead first, the other a bit later with the child. The first adult rides and then waits outside the exit, and then when the adult with child gets to the ride, the child just walks across to the exit and meets them there.