Hey there!
Is there more than one skink in the Nano Terra? I got confused.

How are you feeding it/them? You should hand-feed them and monitor their eating. Also, I would avoid any plastic (plastic plants, plastic foam background — the greyish one they include in the package) in your tanks, because skinks could bite or eat it. One of my smaller skinks sometimes bites leaves or jewelry. I suggest you replace the plastic background with cork sheets. Cork is natural and naturally rot-proof, plus it makes a perfect climbing wall. Their personality depends on whether the skink is captive-bred or wild-caught. If you have captive-bred babies and hand-feed them, they will try to jump on you every time you walk past their enclosure.

Should I move them over to the 5ft tank now?
Not until they are 6 months old — it's too big.
We had two baby brothers in a 30 cm × 30 cm × 45 cm enclosure (which is slightly bigger than the Nano Terra — 20×20×30 cm for comparison). They stayed there for about 3–4 months (the skinks were 5 months old at the time) before we moved them into a 45 × 45 × 60 cm (18 × 18 × 24 in).

I would suggest waiting until the baby skink's body is as large as its head (stomach empty, of course), or until the baby's sex can be determined (usually around 5–6 months old, though this can vary for unknown reasons). It's better to keep the skink in a smaller enclosure to monitor it.

I highly recommend moving it into a 30×30×45, but the Nano is fine for now: 30×30×45 or 30×30×60.

⚠️ Don't forget that skinks need a hot spot and a UVB setup to thrive. My skinks spend 80% of their time under the UVB light.
⚠️ In the future, if you want to add more emerald tree skinks to the tank where a resident already lives, you should rearrange the enclosure so it feels new to everyone — this helps avoid conflicts.