Glad to hear you are riding.
I do not think that crank shorteners need to be limited to those under 6. I have a 10 and 11 year old that are able to use them just fine. The biggest limitation with blocks is that they either cause the pedal to be oriented with the block at the bottom or, if using blocks on both sides of the pedal, they cause a huge decrease in clearance while cornering or even normal pedalling.
Bottom line: figure out what works for you and your stoker. I think that tbe shorteners will work just fine.

WillFam