Hi Kim,
Three granddaughters to sew for, what a treat!
I feel like using the Hopscotch pattern as a base for a dress that you want to look like the Girl on the Go is over complicating things a bit. Not that that’s not what I normally do!
Here’s a few other options to think of:
1: just make the size 8 for the size 6 kid anyway. It’s a t-shirt dress so it doesn’t need to be particularly fitted designwise anyway, and kids grow.
2: use the size 8 pattern but redraw the lines a bit when tracing the pattern to make it a bit smaller, or simply assemble it with a bigger seam allowance.
3: use the Schools Bus tee as your bodice/sleeve option in size 6 then add some length and flare to give it the Girl on the Go shape.
The School Bus tee would be a much easier pattern to start with if you really want to pattern mash as it doesn’t have the surplice front and empire line of the Hopscotch.
I’ll add a link below for all the School Bus customisations as there have been lots of dresses. You might find some inspiration there.
Have fun,
Shelley