I've had the Growing Up products https://www.daz3d.com/growing-up-bundle-for-genesis-8-female-s-and-male-s for awhile now, and I guess it was a co-inky dink that I'd never used it on a Gen8.1 character.

I export all my DAZ characters out via FBX and it's always worked as expected. I export out three versions to actually make things make it easier: one for Wardrobe, another for Hair, and one for the Expressives. But G8.1's Expressives refuses to bake down to the modified shape for the character; the Wardrobe & Hair does. Tried on two different Figures.

The CSV Export rules show that the base figure is marked to Bake.

The G9 version works fine on the few G9 Characters I have, and there doesn't seem to be a Growing Up 8.1 version. 

So is there a setting, technique I need to use?

Not sure what you are saying but I did come across this - https://docs.google.com/document/d/1FMez7Rzs76gKYRZ0_cA5BX1SJVQRE4tNwUw-p3nyMl8/edit?usp=sharing

https://renderguide.com/daz3d-genesis-8-1-overview-features/

Why not just use G8 and avoid all of the problems?

Now THAT sounds like a lot of work! ;) But not out of the question to do if need be.

Basically I'm just trying to figure out why Growing Up is not wanting to co-operate on a G8.1 figure.  My experience with them is that "there is zippo difference between g8 and g8.1". I've not had any problems with G8 characters, but I've a couple new G8.1 figures that I want to make into kids. I'll test more as I suspect it's some morph; as mentioned, the Wardrobe and Hair exports look fine but those only have their respective morphs.

Well the short and sweet answer is Growing Up was not designed to work with the G8.1 figure. lol

Ya, that's the case. G8.1 has different, or rather mixed, morph naming structure. Did a test NOT exporting morphs and that works. So, just have to figure out what's not needed.

All this actually turned out to be a plus. It forced me to deal with the Export Rules for 8.1 and see what was really needed & actually work with 8.1 & the facs system. For example, the Z series (Zeddicuss) don't work with 8.1as expected, but I've a Z_BALBAKE that does, & no idea what figure that came from. Turns out I can prune quite few morphs (as in not export in the first place), and that really saves a lot of time.

Miss Bianca & her Niece.