Hi @lexiegr8,
A warm welcome to the Bunnings Workshop community, it is fantastic to have you with us.
The 140 x 12mm 4.8m Unprimed Solid 321 Lining Boards are intended for use as an internal lining. It is not treated for use outdoors and is susceptible to rot and attack by termites and boring insects. It is not the best thing to use to clad a cubby house.
This 138 x 11mm 5.4m Lining Board H3 Pink Pine FJ Primed 302 LOSP is very similar in terms of profile. It is treated for use outdoors and preprimed for painting with a suitable external paint like Dulux Weathershield. It is a much better choice.
I can't say for certain these lining boards are what they use on flat-pack cubby houses, but it is suitable for use as external cladding for a cubby house.
Let me know if you have any further questions.
Jacob
