I used the very same model you posted, and they are quite effective in helping a kid use a tandem. In fact, my daughters never used a stoker kit, but started using the regular rear seat (18" compartment) with a low seatpost and crank shorteners.
Regarding their use, they don't work like wood blocks. Wood blocks will raise the effective crankset, but your child will still be pedalling a 170-mm or 175-mm circle. So she'll be able to reach the bottom of the stroke, but with her short legs, she'll eat her knees at the top of the stroke. Crank shorteners, on the other hand, make the cranks shorter, so she'll be pedalling a 110-145 mm circle. A child bike and a Trail-a-Bike have 135 or 145-mm cranks (I'm going by memory here), and considering the length of her legs, you should ideally aim for something like that, which would mean the second hole from the top (i.e. 2nd largest circle available with the shorteners). I had to start my then 7.5 year-old daughter from the third hole, but within a few months, I moved her to the second hole and I removed the shorteners when she was 9.5.
You might also want to look at the seatpost. The standard seatpost supplied by Co-Motion has a gracious curve, but there has to be at least 10 or 15 mm of seatpost showing. The LBS swapped that seatpost with a Burley, and with it no seatpost needs to appear. In other words, I gained an extra 10 or 15 mm there.